Before deciding on a multi island trip keep in mind a few things:
1. Do you have enough time? If you are only spending 5 days in Hawaii it's best to stay on one island. If you must, then do Oahu for 2 days and then pick any island for the remainder 3 days. This is not recommended.
2. For 10 day stays, the most popular combos are; if you really want to relax do a 5 - 5 (5 nights on each island), using Kauai and Maui. That way you can really relax and experience each island. If you want more activities and sightseeing then do a 3 Island combo 3-4-3
3. Don't over plan your trip by planning too many activities.
4. I recommend at least 3 nights per island minimum except for Oahu.
Some encouraging news, getting between the islands is getting less and less expensive with GO airlines slashing airline prices and a ferry service set to enter the market in 2007. The ferry will let you rent a car on any island, drive onto the ferry and the dirve it off!