Spent many a childhood holiday in Guernsey, personally I love the place. It's got some lovely beaches, and plenty of wartime stuff to explore - lots of bunkers/fortifications etc. St Peter Port is a nice quaint town, the creperie always used to be worth a visit.
The British hillclimb championship visits there every summer (July this year:
http://www.top12runoff.co.uk/) which should be spectacular - I've cycled the route many times and it's a killer on a bike at least. There's often sand racing at Vazon bay on the west side of the island.
Climate is usually pretty good, more like France than England funnily enough
Alternatively, France is on your doorstep and there are always the other islands to explore...
Jamie