All the best beaches are in the West with my fav being Plemont.
Other great beaches (there are loads):
St Ouen (surfing & other activities)
Portelet
St Brelade (nice family beach with kayaks etc and a good base to paddle round to some smaller bays)
There aren’t many campsites on the island so I’d probably do Daisy Cottage (North West near Plemont etc) & Rozel (North East near the Zoo, Gorey Castle). I’ve not stayed at either as I lived there so others may be able to comment better.
Some good country lane cycling to be found around each and some nice routes to get between the 2 (country lanes & 15mph ‘green’ lanes), with stop offs like Devils Hole, Bonne Nuit or Bouley Bay.
PM if you want more info on anything & I’ll see what I can help with.