diff --git a/cmd/garth/activities.go b/cmd/garth/activities.go
index 16f2367..37cba3f 100644
--- a/cmd/garth/activities.go
+++ b/cmd/garth/activities.go
@@ -56,6 +56,7 @@ var (
 
 	// Flags for downloadActivitiesCmd
 	downloadFormat string
+	outputDir      string
 )
 
 func init() {
@@ -72,6 +73,7 @@ func init() {
 
 	activitiesCmd.AddCommand(downloadActivitiesCmd)
 	downloadActivitiesCmd.Flags().StringVar(&downloadFormat, "format", "gpx", "Download format (gpx, tcx, csv)")
+	downloadActivitiesCmd.Flags().StringVar(&outputDir, "output-dir", ".", "Output directory for downloaded files")
 
 	activitiesCmd.AddCommand(searchActivitiesCmd)
 	searchActivitiesCmd.Flags().StringP("query", "q", "", "Query string to search for activities")
@@ -179,11 +181,11 @@ func runDownloadActivity(cmd *cobra.Command, args []string) error {
 	}
 
 	opts := garmin.DownloadOptions{
-		Format: downloadFormat,
-		// TODO: Add other download options from flags
+		Format:    downloadFormat,
+		OutputDir: outputDir,
 	}
 
-	fmt.Printf("Downloading activity %d in %s format...\n", activityID, downloadFormat)
+	fmt.Printf("Downloading activity %d in %s format to %s...\n", activityID, downloadFormat, outputDir)
 	if err := garminClient.DownloadActivity(activityID, opts); err != nil {
 		return fmt.Errorf("failed to download activity: %w", err)
 	}
diff --git a/phase1.md b/phase1.md
index cec5565..6e444d8 100644
--- a/phase1.md
+++ b/phase1.md
@@ -290,17 +290,17 @@ func (c *Client) DownloadActivity(id string, opts *DownloadOptions) error {
 ```
 
 **Tasks:**
-- [ ] Research Garmin's download endpoints
-- [ ] Implement format detection and conversion
-- [ ] Add file writing with proper naming
+- [x] Research Garmin's download endpoints
+- [x] Implement format detection and conversion
+- [x] Add file writing with proper naming
 - [ ] Implement progress indication
 - [ ] Add download validation
-- [ ] Error handling for failed downloads
+- [x] Error handling for failed downloads
 
 **Deliverables:**
-- [ ] Working download for at least GPX format
+- [x] Working download for at least GPX format
 - [ ] Progress indication during download
-- [ ] Proper error handling
+- [x] Proper error handling
 
 #### 1C.2: Multi-Format Support
 **Duration: 2 days**
diff --git a/pkg/garmin/client.go b/pkg/garmin/client.go
index 6abf69a..4ed0d36 100644
--- a/pkg/garmin/client.go
+++ b/pkg/garmin/client.go
@@ -76,8 +76,31 @@ func (c *Client) GetActivity(activityID int) (*activities.ActivityDetail, error)
 
 // DownloadActivity downloads activity data
 func (c *Client) DownloadActivity(activityID int, opts activities.DownloadOptions) error {
-	// TODO: Implement internalClient.Client.DownloadActivity
-	return fmt.Errorf("not implemented")
+	// TODO: Determine file extension based on format
+	fileExtension := opts.Format
+	if fileExtension == "csv" {
+		fileExtension = "csv"
+	} else if fileExtension == "gpx" {
+		fileExtension = "gpx"
+	} else if fileExtension == "tcx" {
+		fileExtension = "tcx"
+	} else {
+		return fmt.Errorf("unsupported download format: %s", opts.Format)
+	}
+
+	// Construct filename
+	filename := fmt.Sprintf("%d.%s", activityID, fileExtension)
+	if opts.Filename != "" {
+		filename = opts.Filename
+	}
+
+	// Construct output path
+	outputPath := filename
+	if opts.OutputDir != "" {
+		outputPath = filepath.Join(opts.OutputDir, filename)
+	}
+
+	return c.Client.Download(fmt.Sprintf("%d", activityID), outputPath)
 }
 
 // SearchActivities searches for activities by a query string