Yes, you can sync a playlist to the watch so it can play it independently: https://support.apple.com/en-us/HT204691
Edit - nevermind - looks like only a single playlist can be synced. That's a pretty major flaw in the system.
BTw in practice I haven't found charging an issue on my 0th gen apple watch and I use it to track both exercise and sleep. It's not "fire and forget" but neither was the iphone for a long time either.