Except for the major museums, like the Met, the Museum of Natural History, the MoMA, or the Guggenheim. most of the smaller museums are not going to be crowded unless it’s a special exhibition or a free/reduced-rate day. I’ve been in some museums where I could count the people there with me on one hand.
I did a bunch of posts in the NYC in August thread which cover more of what you are looking for, including neighborhood walking tours. You’d probably like Governor’s Island.
Other walking neighborhoods would be Greenwich Village and the East Village, but they are built more around small shops and restaurants. Haven’t been to either in a while and I really can’t say after COVID what is still around. I noticed a few of my favorite restaurants in those areas have closed.
I would say that if you just aim to walk around, pick a starting point and an ending point, plan to hit an interesting place or two on the way and walk. Have a general idea of what’s in the area and what route to take. Some people might tell you that there are no bad walks in Manhattan, but there are definitely stretches of blocks where the storefronts are empty because the landlord is trying to put together some sort of Hudson Yards-type deal or the rents are so high that only major chains (Starbucks, Duane Reed, Citibank) line the street.