1. you can find very good all inclusive deals online, better choose a 4-star hotel to stay. Usually the all-inclusive package in Varadero is the cheapest.
2. You can take bus from Varadero to Havada (about 100 KM), stay in Havada for one or two nights (pay extra money for the hotel). Taxi in Havana is pretty cheap.
3. Join 1 or 2 day-trips from Varadero (not cheap).