I find that turning on Airplane Mode overnight substantially extends my battery life, perhaps from 3 to 5 days. It also prevents me from being awakened by notifications, which seem to come through regardless of DND mode on my iPhone. It's also easy to trigger: just hold the left side button for a couple seconds.

I would pay $75 for someone to replace the battery in my Pebble and maintain water resistance. I don't trust myself to do it, but eventually I may try. My current fallback plan is a Fossil hybrid smartwatch, which is nicer looking, but has a worse UI/UX.