I've had a Pebble smart watch ever since their Kickstarter and I use it every day. My main use is to get email notifications and also notifications telling me who is calling me on my phone (so I can ignore them if I don't want to talk to them ). I do have a Flappy Bird game on the watch and it seems like several simple games could be done for that phone, which has an SDK.
Game and Watch is an obvious way to go. Maybe also early phone game or even earlier console games like the Atari 2600. Back when Pixels were king. Personally, I'm waiting for the Firefox OS watch! I have grown to dislike Samsung, so I don't want their big ugly watch. The Pebble just put out version 2.0 of their SDK. My original Pebble cost $99 but I think the cheapest now is $150 but they have flashier ones too. Mine is a nice black tasteful watch that don't attract a lot attention.
Right now the Pebble store has about 1,000 apps in it!
It looks like fun but I'm having way too much fun with Firefox OS (and Construct 2) to do much.
For more Pebble info, check out https://getpebble.com/