We used to go to Orlando and do the one week offsite/one week onsite thing. While offsite, we went to US, IOA, SW, BG, and Wet N Wild. For the last few trips, we have stayed at WDW and only taken one day per trip to do something offsite. Last year we went to KSC and had a really great time.
Our two favorites are SW and BG. Both myself and DS (17) are the "coaster junkies" in our family. We prefer BG to IOA. One reason is that there is something for everyone to do there. We also think the grounds/landscaping is much better at BG. Everyone in our family loves animals, so that's probably why SW and BG top our list. AK is actually one of favorite WDW parks.
This year I asked our boys to list the three things other than WDW they would both like to do and one of them listed BG, SW, and Wet n Wild. The other listed BG, IOA, and Wet n Wild (we love the water parks, too!).
It usually takes us about 1.5hrs to get from our hotel to BG. It is an easy drive over and back. We can easily do it in a day.
DH and I are still deciding what to do on our "extra" day this year. I have a feeling BG will win!