[femto]

Railway preservation (full size, not model). It looks crowded when a steam train is running and the moths gather around. The reality, when the trains are not running, is typically quite different, with a small dedicated group. If a place looks too crowded, pick a smaller museum.

Think of all the jobs that have to be done to run a railway and you will be able to find a museum that does it: heavy maintenance, boiler work, fitting and turning, blacksmithing, woodwork, upholstering, painting, catering, engine driving, fireman, signalling, customer service, ...

It's a great way to meet people, learn new skills and work with physical things.

[bluGill]

Only problem is this really depends on where you live. There is a nice museum 45 minutes from me - far enough that it is hard to get there for a quick evening after work...