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P&O are very good and have gym facilites etc. There are usually few children on board during term time but you rarely see them as they have their own childrens facilities which are very good. If you don't want kids at all then Arcadia and Artemis are adult only ships.
If you fancy sailing in warmer waters try Thomson Calypso it is Adult only and they have some nice itinerarie.
Hi P and O have excellent gym facilities, and fitness activities, with the added bonus of child free ships, ARTEMIS AND ARCADIA. If you chose another ship, remember its half term in October.