The Trotsky museum is well worth a visit. It opens at 10am until 6.30pm every day except Mondays. At 10am on a Tuesday, we were the only visitors and we had the place to ourselves.
There is a museum containing images and information on Trotsky, but most interesting is his house, still in the exact way it was left. I found it quite uncomfortable, as the bullet holes in the wall are close enough to touch, and his study where he was assasinated still has his glasses on the desk. Very interesting.
The house of Frida Kahlo is amazing. As well as showing her works, you can walk around the house. It is bright, vibrant, and fascinating. It wasn't too crowded and opens at 10am.