You can delete a activity from your 'Activities' by following these steps:
Via desktop or laptop computer:
- Log in to your account on https://www.alltrails.com
- Hover over 'Saved' in the header and click on 'Activities'
- Click on the activity that you would like to delete
- Click on 'Edit' and under the edit screen click 'Delete'
Via iPhone and Android app:
- Open the AllTrails
- Tap on 'Saved' on the bottom navigation bar
- Select 'Activities' from the tabs at top of the screen
- Tap on the overflow menu in the upper right corner and select 'Edit'
- Tap the minus icon for the activity you wish to delete