On the Apple Watch, Things lets you see tasks in the Today folder in Glances. You can create new task from within the Apple Watch app by clicking the Plus button. You can add notes or a due date from within the app.
Cost: iOS app $9.99
Toodledoo shows you Hot tasks in Glances. In the Apple Watch app, you can click a task to show details, then force-touch on the task to complete or delete it. Toodledo also has a web client that you can use to add items.
Cost: iOS app $3.99
Todoist shows you scheduled tasks in Glances, but you can see all your lists and list items in the Apple Watch app. You can also add tasks by force-touching the app on the Apple Watch and selecting Add Task. Because Todoist has a channel in IFTTT, I created a recipe to copy any shopping list item from the Amazon Echo to the Todoist Inbox. That way, I can see my shopping list on my watch, eliminating the juggling I do trying to see the list on my iPhone while pushing a grocery cart.
Cost: iOS app free, with an optional in-app purchase $28.99 for the yearly premium subscription. Premium features include reminders, task notes, and the ability to attach files.