Oohh lots!
I would deffo reccoment that you get Angela Birds guide book (she has a good website)
Brem sur mer is a nice town, and good beach, Jard sur mer, and little further south is nicer , I think. St Gilles Crox de ville is fabbo and there is a great fish resturant in the square near the church, there is a mini fair at the end of the Harbour and a nice carosel on the bridge.
The festival in Challans is great and is free and the whole twon dresses up, well worth going to. The caste in Apremonyt is worth a visit, and there is a nice creperie at the bottpm of the castle wall....you can also hire pedelos and go up the river , which is a hoot.
the drive across the causeway to norimoutier is worth doing and there is a first rate water park there.
never done the velo rail, but it is supposed to be good
You can hire 4 man 'bikes' to go up and down the sea front at St Jean
fantastic hypermarche at les Sable d'ollone (how sad am I?'
and the zoo there is good for the kids