Jersey is the busiest with good shops, marvellous seafood a good Marina. It also has an excellent zoo that is dedicated to conservation. It was started bu Gerald Durrell.
Guernsey is smaller, but has reasonable shops, good food, a small zoo. Gorey is picturesque with its castle, pottery, harbour.
Alderney is small. 5miles long. The only town is St Anne's which has one street of shops.
Sark is a different world! No motorised transport, only approached by boat and then a long walk up the hill. Nice walks and wild flowers.
You can hire bikes on all the islands and cars on Guernsey and Jersey