Glencoe visitor’s centre for indoors (and some outdoors). Signal Rock/An Torr is a lovely spot for a wander.
You can do a lot of sightseeing without going far from the road, I almost always stop a couple of times. Actually worth stopping on the a82 as it goes across rannoch moor towards glencoe (probably best once it gets all lochan-ey) and then to check out Buachaille Etive Mor, and the big waterfall halfway down the glen, basically anywhere you fancy the look of. Obviously tons of places to walk/climb if you’ve got the time. Oh and the views down loch linnhe near the ballachulish bridge can be absolutely incredible, especially later in the day, never seen a photo that does it justice.
Green Welly Stop I do not understand- used to always stop there on the motorbike as it’s a traditional gathering spot, but it’s basically a very small motorway service station.
When you’re coming back, if you like driving go the long way around loch leven, via kinlochleven- the german road along the north shore is fantastic to drive and there’s some fantastic views down the loch, from both sides. Probably adds 20 minutes or so to the drive, bit longer if you stop for the views. Probably not worth stopping in the town in all honesty, you can go up the Way a little and check out the pipelines and tailrace, maybe the ice factory… But that’d be way low on my list.