singular or plural: objects following "sometimes"
a) I sometimes go to a museum.
You would say this when you often go to one particular museum (e.g., the Museum of Modern Art). You can also say this when you go to any museum on a particular occasion.
When I am depressed, I sometimes go to a museum or an empty theater.
The other one
b) I sometimes go to museums.
can be said when you visit multiple museums, both in a single occasion and in different ones.
When I go to NYC, I sometimes go to museums, parks, and libraries.