Jetboy, never fear about towels, as they do provide them for the pool as well as the cabin. There should be no charge for the gym - I am not sure about any sauna that there may be. Hot tubs, no charge. Other spa services, like massages, haircuts, herbal wraps, etc. will cost extra; if you are interested in any, check with the spa shortly after boarding for prices and to sign up (sea days are very popular and fill up fast.)
The food should not be bad. I personally don't think it's quite up to RCCL standards, but it should be fine. From all reports, good idea to have budgeted for the specialty restaurants, though.
Have a safe journey - and please remember to just relax and enjoy. If it's your parents' first cruise, all the better. If not, a good attitude makes any cruise a good one!
Have a wonderful time!