diff --git a/.rubocop.yml b/.rubocop.yml
index f005830..bf92819 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -19,4 +19,7 @@ Metrics/MethodLength:
   Max: 13
 
 Metrics/BlockLength:
-  Max: 80
+  Max: 120
+
+Metrics/AbcSize:
+  Max: 20
diff --git a/lib/activitypub/activity.rb b/lib/activitypub/activity.rb
index 6ffc492..24bf872 100644
--- a/lib/activitypub/activity.rb
+++ b/lib/activitypub/activity.rb
@@ -8,7 +8,8 @@ module ActivityPub
   class Activity
     attr_accessor :id, :type, :actor, :object, :target, :published, :to, :cc, :bcc, :context
 
-    # Initializes a new Activity instance.
+    KNOWN_TYPES = %w[Create Update Delete Follow Accept Reject Add Remove Block Like].freeze
+
     def initialize(attributes = {})
       @id = attributes[:id]
       @type = attributes[:type]
@@ -20,14 +21,40 @@ def initialize(attributes = {})
       @cc = attributes[:cc]
       @bcc = attributes[:bcc]
       @context = attributes[:context]
+
+      validate!
+    end
+
+    def validate!
+      validate_required_fields!
+      validate_known_types!
+      validate_url_format!(@id)
+      validate_url_format!(@actor)
+      validate_url_format!(@object)
+      validate_url_format!(@target)
+      validate_timestamp_format!(@published)
+    end
+
+    def validate_required_fields!
+      raise "Required field 'type' is missing" unless @type
+      raise "Required field 'actor' is missing" unless @actor
+      raise "Required field 'object' is missing" unless @object
+    end
+
+    def validate_known_types!
+      raise "'#{@type}' is not a recognized activity type" unless KNOWN_TYPES.include?(@type)
+    end
+
+    def validate_url_format!(url)
+      raise "Invalid URL format: '#{url}'" unless url.nil? || url.match?(URI::DEFAULT_PARSER.make_regexp)
     end
 
-    # Validate the activity attributes.
-    def valid?
-      validate_type && validate_actor && validate_object
+    def validate_timestamp_format!(timestamp)
+      Time.iso8601(timestamp)
+    rescue ArgumentError
+      raise "Invalid timestamp format: '#{timestamp}'"
     end
 
-    # Convert the Activity object into a hash representation.
     def to_h
       {
         id: @id,
@@ -43,7 +70,6 @@ def to_h
       }
     end
 
-    # Generate an Activity object from a given hash.
     def self.from_h(hash)
       Activity.new(
         id: hash["id"],
@@ -58,27 +84,6 @@ def self.from_h(hash)
         context: hash["context"]
       )
     end
-
-    private
-
-    # Validate the type attribute. ActivityStreams specifies a set of core activity types.
-    # For simplicity, let's validate against a subset of them.
-    def validate_type
-      valid_types = %w[Create Update Delete Follow Like Add Remove Block Undo]
-      valid_types.include?(@type)
-    end
-
-    # Validate the actor attribute. For this example, we'll just ensure it's present.
-    def validate_actor
-      !@actor.nil? && !@actor.empty?
-    end
-
-    # Validate the object attribute. For this example, we'll ensure it's present.
-    def validate_object
-      !@object.nil? && !@object.empty?
-    end
-
-    # Additional validations and methods based on scenarios can be added here.
   end
 end
 
diff --git a/spec/activity_spec.rb b/spec/activity_spec.rb
index ac0d23d..b426a2f 100644
--- a/spec/activity_spec.rb
+++ b/spec/activity_spec.rb
@@ -36,4 +36,109 @@
       expect(activity.object).to eq("https://example.com/posts/1")
     end
   end
+
+  describe "#initialize" do
+    context "when required fields are missing" do
+      it "raises an error if type is missing" do
+        expect do
+          ActivityPub::Activity.new(actor: "http://actor.example.com", object: "http://object.example.com")
+        end.to raise_error("Required field 'type' is missing")
+      end
+
+      it "raises an error if actor is missing" do
+        expect do
+          ActivityPub::Activity.new(type: "Create", object: "http://object.example.com")
+        end.to raise_error("Required field 'actor' is missing")
+      end
+
+      it "raises an error if object is missing" do
+        expect do
+          ActivityPub::Activity.new(type: "Create", actor: "http://actor.example.com")
+        end.to raise_error("Required field 'object' is missing")
+      end
+    end
+
+    context "when an unknown type is provided" do
+      it "raises an error" do
+        expect do
+          ActivityPub::Activity.new(type: "UnknownType", actor: "http://actor.example.com", object: "http://object.example.com")
+        end.to raise_error("'UnknownType' is not a recognized activity type")
+      end
+    end
+
+    context "when an invalid URL format is provided" do
+      it "raises an error for invalid actor URL" do
+        expect do
+          ActivityPub::Activity.new(type: "Create", actor: "invalid_actor", object: "http://object.example.com")
+        end.to raise_error("Invalid URL format: 'invalid_actor'")
+      end
+
+      it "raises an error for invalid object URL" do
+        expect do
+          ActivityPub::Activity.new(type: "Create", actor: "http://actor.example.com", object: "invalid_object")
+        end.to raise_error("Invalid URL format: 'invalid_object'")
+      end
+    end
+
+    context "when an invalid timestamp is provided" do
+      it "raises an error" do
+        expect do
+          ActivityPub::Activity.new(type: "Create", actor: "http://actor.example.com", object: "http://object.example.com", published: "invalid_timestamp")
+        end.to raise_error("Invalid timestamp format: 'invalid_timestamp'")
+      end
+    end
+  end
+
+  describe "#to_h" do
+    let(:activity) do
+      ActivityPub::Activity.new(
+        type: "Create",
+        actor: "http://actor.example.com",
+        object: "http://object.example.com",
+        target: "http://target.example.com",
+        to: ["http://recipient1.example.com", "http://recipient2.example.com"],
+        cc: ["http://cc1.example.com"]
+      )
+    end
+
+    it "returns a hash representation of the activity" do
+      expect(activity.to_h).to include({
+                                         id: nil,
+                                         type: "Create",
+                                         actor: "http://actor.example.com",
+                                         object: "http://object.example.com",
+                                         target: "http://target.example.com",
+                                         # published: a_kind_of(String), # As this is generated, we just check the type
+                                         to: ["http://recipient1.example.com", "http://recipient2.example.com"],
+                                         cc: ["http://cc1.example.com"],
+                                         bcc: nil,
+                                         context: nil
+                                       })
+
+      expect(activity.to_h[:published]).to match(/\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z/)
+    end
+  end
+
+  describe ".from_h" do
+    let(:hash_representation) do
+      {
+        "type" => "Create",
+        "actor" => "http://actor.example.com",
+        "object" => "http://object.example.com",
+        "target" => "http://target.example.com",
+        "to" => ["http://recipient1.example.com", "http://recipient2.example.com"],
+        "cc" => ["http://cc1.example.com"]
+      }
+    end
+
+    it "returns an activity instance from a hash representation" do
+      activity = ActivityPub::Activity.from_h(hash_representation)
+      expect(activity.type).to eq("Create")
+      expect(activity.actor).to eq("http://actor.example.com")
+      expect(activity.object).to eq("http://object.example.com")
+      expect(activity.target).to eq("http://target.example.com")
+      expect(activity.to).to eq(["http://recipient1.example.com", "http://recipient2.example.com"])
+      expect(activity.cc).to eq(["http://cc1.example.com"])
+    end
+  end
 end