Had 2 weeks at Varadero recently and did a 2 day trip to Havanna. Just wandered, looked and drank in the main - it's a nice place. Hemmingway's old hotel - Ambos Mundos or something - is a great spot for a mojito, listening to the pianist and watching the world go by.
There's a fab 'House of Chocolate' close by which does the most amazing chocolate drinks - like a milkshake - and they cost about 50p. Well worth ducking out of the heat for.
The Floridita - Hemmingway's bar feels like a 'must go' even if you just have a drink and have your photo taken by the bronze Hemmingway still sitting at the bar.
The Camera Obscura is brilliant, especially if you've never done one before anywhere. It costs a couple of quid and won't take up more than half an hour of your time.
I wondered why on earth we were going to the 'world famous' Tropicana night-time cabaret show thing but it was surprisingly good. Your entry includes half a bottle of rum and a can of Cuban cola - this may have contributed to my enjoyment of the evening!
Bogadito del Medio (sp) and the museum of the revolution. Go dancing and really feel like a gringo. Go down to Trinidad and up into the Escambre Mountains. Cristophe Colon cemetary. Try to meet some Santerio worshippers. Meet Cubans they are a fantastic people.