Thormanby Island on the Sunshine Coast has some of the BEST beaches in British Columbia. There's no resort, store, or restaurant on the island but there is a small campsite at Buccaneer Bay Provincial Park. Make sure you pack snacks, suntan lotion, and lots of water!
Thormanby Island is only accessible via boat. If you can't provide your own, there are a number of water taxis that are happy to ferry you back and forth!