If you are young or even look young, you will have people UP YOUR ASS all the time, "Hey, where you from ?" means, "I am going to get close to you and annoy you." I had to just keep walking so I wouldn't freak out on anyone, that's why I look like this in these photos (see above). If you are old or look old, you won't get hassled as much. I've never enjoyed being rude as much as I enjoyed being rude in Old Havana.
You can also walk around and not be hassled, folks mainly approach you outside your hotel, but after you are clear, you're alright to head out without being bothered.
The National Gallery is a great way to kill an hour and a half, really enjoyed alot of the work I saw there, next trip I'll check theInternarional Collection, which is a separate building, I just wanted to see some Cuban art that wasn't a painting of an old wrinkly guy holding a cigar or Che Guevara or Castro.
If you see some musicians playing STOP and check them out, it is impossible to find a bad live band in Old Havana, stop and check out every band you see. I don't know about buying cd's, the live sound of Old Havana is probably hard to capture in a studio.
The air in Old Havana is polluted, it's not horrible, but you may not be used to poor air quality so don't exert yourself too much, take breaks from walking, especially from walking in the sun after breathing the air. Yes, we love taking pictures of the old cars, but it comes at a price.
You can totally crash the mini bar in a Cuban Hotel, it's that cheap, you can also buy beer in vending machines.
I would go back to Cuba next year for a vacation, no problem. It was great.