One of the best presents you can buy for a Child's Christmas is a Trampoline. My kids play on their's all year round. This Sportspower 8ft Trampoline with Folding Enclosure is ideal and won't break the bank at £89.99 from Argos.
The Sportspower 8ft Trampoline has a galvanised steel Quad Lok style frame, so should be weatherproof and easier to put together.
I like that this one has an "Easi Store" Enclosure so that you can fold it down when it's windy.
Note you'll need two hours to put this together and a helper too.