We found Havana to be a wonderful old city with lots of music and local colour and there are beaches nearby.
Santa Clara was interesting and has the "Che" museum and you can visit the train that Che attached and defeated.
Trinidad was a nice old city but rather over-hyped.
As for great beaches, you may have to ask someone else.
We were there over 10 year ago and I believe that there have been a number of improvements since.
We stayed on Cayo Coco, a nature reserve of the coast of mainland Cuba, beaches were lovely, from the we visited Havana for the weekend, Trinidadian City for the day and the town of Moron, it is one of the few places that we've visited that we would be quite happy to return to. The hotel we stayed at was the Melia which had suites situated on the lagoon.
I was there in 2008 - I wanted to go and see it before the Castro's died and it became a Disneyland - I'd recommend it but a few things (bearinig in mind it was 5 years ago)
1/ Dont go with Air Cuba if you can help it - it's like 1970's Aeroflot - no inflight entertainment, very basic - I think Virgin flys there too go with them if you can.
2/ You'll have to change money at the airport - you'll be tired, count your change carefully I was short changed!
3/ All the hotels will be all inclusive and the food will be OK adequate - however the resaurants outside are very good and not that expensive do go to one or two despite missing 'already paid for' food.
4/ Be careful of food on excursions - I got very sick almost certainly from food on one of the sailing trips - watch what you eat on such things and take some emergency diacalm type remedies just in case - on the plus side I can say Cuba's health service is as excellent as its reputation!
5/ Hotels - I guarantee you will find some small bug - a blown light bulb, a lock not working, something like that - if they fix it will depend on when they can get into another room to replace the part! - There isn't really any reliable maintenace companies so they do the best they can so you need a little patience with such things.
The beaches at Varadero are beautiful white sand - gorgeous and within striking distance of a trip to Havana which I'd also recommend. I also went on a diving trip to the Bay of Pigs which was also gorgeous.
If you want a no flaws luxury experience it's probably not for you but if you want a beautiful place warts and all that's bit of an experience and you're up for that -go and go now