Thun, Spiez or Brienz for a day trip

IMHO, Brienz. You can get to or from via a nice boat trip from Interlaken. The boat goes past Geissbach Falls, very pretty falls, and you could take a 1 hour hike there and catch the next boat. While in Brienz, you can take the Rothorn Bahn (steam powered cog railway) up to a mountain top. There's a nice waterfront to walk. And a lot of woodcarving shops. It may or may not interest you, but you could take a short bus ride to the Ballenberg Open Air Museum. That could take quite a few hours, you could find them on the web and see if it's worth the time for you. My wife and I love it.

Spiez or Thun are not that remarkable, IMHO, but you can take a boat from the Interlaken West train station to Thun.

And put Thun as your second choice - Spiez is not that walkable, unfortunately.

Thanks for the replies!
If I want to see the town for the town itself, not activities outside the town, would Brienz still be the choice on these two lakes?

Brienz is scenic, and I also prefer Meiringen just up the track. Or jump off the lakes and take the train from Interlaken Ost, Brienz and Meiringen toward Luzern and hop off at the summit of the Brünig Pass just a few minutes later.

Thanks everyone! We decided to go to Brienz, and ended up going to the Ballenberg Open Air Museum. It was wonderful! So much to see there. I would recommend starting from the east end and wandering west, because it is more downhill that way, and use the map they provide and circle things on it you want to see. We got a little tired from the heat and didn’t pay enough attention to the informative map they give you, and we missed a lot of things that would have interested us. We did chat with a very sweet woodcarver person and bought a few ornaments from him. I really liked that the people there really care about the crafts and culture represented at the museum.
Afterwards we took the bus back to Brienz and walked on the lakeside path mentioned here, and to the old street named Brungasse, and to the old church on the hill. I jealously watched people enter the water for a swim at the lakeside.
Thanks travelerguy for suggesting Brienz.
When we took the train past Thun yesterday, it looked quite built up with compared to Brienz, because there is countryside right next to Brienz’s centre, so I am glad we went to Brienz!

so glad that worked out for you. I think you made the right decision.