Waterpark lowdown from a local!
Gateway cafe is fine, but packed over the summer and food is a bit pre-prepared cafe stuff.
Lakeside brasserie has the best burgers, ribs and other bbq fare! Also does roasts at the weekend. When weather is nice the terrace is a great place to watch waterskiing.
4 Pillars hotel food is v good, quite expensive, wife says spa also good, but often full of weddings.
Best pub nearby is the Wild Duck in Ewen (GL7 6BY) – great food and lovely location – I would go there for a family meal.
Best pub further away is the Tunnel House in Coates.
Food at the polo club in Ciren is not great, but polo watching is fun (usually free or £5 per car to get in).
Corinium museum in Ciren is worth a visit, as is the Roman amphitheatre. Shopping ok, farmers market can be good, but best one is in Stroud on a Saturday. The Lido in Ciren is great on a sunny day, and you can take a bbq. Walking in Ciren park is also lovely, but can be busy and ++ dog poo.
Best MTB riding is over toward Stroud and up to Cheltenham as water park is vv flat. Loads of roadie routes around though (not one myself, but friends are)
Water based fun = cable wakeboard is worth a go, boat based skiing is more expensive, stand up paddle boarding is good family fun.
Oasis in Swindon (indoor pool + waves with slides) looks tired, changing rooms a bit grim but is great fun for kids on a rainy day.
Hope that helps!