Avoid going to the family home of one of the group members. Even if this is the cheapest option, you might find that you can’t properly relax. The "guests" would feel less at ease, be afraid of breaking something and maybe feel indebted.
Pick somewhere neutral that meets all the criteria you set: sea, mountains, countryside, holiday club, self-catering, B & B, hotel, etc. Find somewhere big enough so everyone can have their own space and privacy (one room per couple or single person) and enough bathrooms to avoid queuing!