I wouldn't recommend more than one island in Hawaii unless your just looking for a taste of the Islands.
Maui has many great beaches, restaurants, things to do and see.
Kauai is considered the nature island. The Nai Pali Coast is amazing.
Oahu is pretty touristy but generally cheaper. You could see Pearl Harbor and visit the North Shore.
Big Island - check out the volcano its active right now..
The links above should give you a good flavor for the
Hawaiian Islands.
Where do you live? I would consider flight times and travel costs since getting to Hawaii from the East coast of the US takes alot more work for a 7 day trip.
I would look into the
Caribbean Islands or
Bahamas in that case.