If one is "into" museums, virtually every major European city has lots of them--one can spend years exploring a major one, and just scratch the surface.
On the other hand the "street scene" excitement of places like Paris and London can whet one's appetite to return many times. If you are "into" beautiful scenery (and are also a train buff) Switzerland, can keep you occupied for many years.
[Side comment: some people love shopping, and this endeavor can be reason to to return ti visit some cities multiple times.]