I've photographed there a few times, so I hope the following will help.
More than most places I go, the opportunity seems to be in black & white and in colour, especially in Havana. How you manage this is clearly important- I decided to have b&w days and colour days rather than hop from one to another, which I don't always handle well. Also I didn't want to do the large amount of walking that Havana entails whilst carrying two medium format systems. In my view the b&w opportunity is more likely to be people orientated and will benefit nore from the speed and precision of the 35mm slr, whereas I haven't felt at all disadvantaged by using MF for all the colour work.
In Havana, the northern part of Habana Vieja is a little overblown with tourism and repainted ice-cream colours now. The Prado(Paseo de Marti) still has a lot to offer, as has the area immediately to the west. The further you go west here the more battered it becomes. There has been quite a bit of restoration along the Malecon, but its still a must see. Late pm is best. There are areas off the beaten track where probably a little more care is needed but are neverthelessrewarding. The Avenida Maximo Gomez (Monte) , the Ave. Simon Bolivar and the touch of reality that lies between them are interesting, I also like the streets north of the station and the beat-up southern part of Habana Vieja. You'll notice very quickly that decay has reached crisis point. Many of the best buildings are being restored or are being knocked down before they fall down. In Havana; in fact in Cuba, restoration is not your friend and its frightening to note that of the twelve photographs I've posted on my web-site (www.photography001.com) from my first visit in 2001, only four were still available when I last visited in November. Even the cars have seemed to deteriorate to the point that many of them are now ramshackle not beautifully preserved, so believe me you're not going a day too soon.
I haven't seen much countryside in Cuba that I'd like to photograph, but there are some beautiful towns. In a week, a circuit of Cienfuegos, Trinidad, Sancti Spiritus, and Remedios would make a lot of sense. The driving is a nightmare incidentally, scarcely any road signs. Mosyt people go to Trinidad and again just in the last few years the signs of mass tourism are clearly there. Just get 200 yards from the main square and the "market" and it'll seem a lot better.
Presumably you've read that the food's bad. Beers decent. Don't touch the wine. Drink bottled water.
A great place, enjoy it.