Hi, many hotels in Varadero offer day pass options. Prices vary from about $15 per day to over $100. The day is usually from about 9am until 7pm.
You would not be able to reserve these day passes on the internet. You would have to show up at the hotel and ask for availability and prices. If you are in Havana, you can go to any large hotel (Habana Libre is good) and ask at the Cubanacan or Cubatur desk (beside the lobby) to see if they can book you a day pass reservation. Sometimes they have deals, especially if you are traveling during low season (summer).
The hotels near the town of Varadero are cheaper, usually below $50 per day. Hotels between the Puntarena Hotel and the Tuxpan Hotel are usually the cheapest. These hotels are older and less quality, so the prices are low. Many Cubans go to these hotels just for the day. The hotels sometimes have cheaper rates for Cubans and higher rates for foreigners. Higher end resorts might not offer day passes.