Winduck, try turning off all the driving aids and using a manual transmission. Huge difference.
I can spend hours with this game, from car shopping to real estate to modifying my rides to just sitting in each vehicle looking around the interior. Just don't burn out the electric window motors. Plus there's the ability to drive over every inch of the island (who needs roads?) looking for cool places to set-up photo shoots (an Astin-Martin flying off the top of a secluded waterfall near Aeia is one of my favorite shots). Don't get caught by your Mom when you're photographing your female character riding her Ducati in those little tight surfer shorts you bought her at the Echo store down in Waikiki.
And that's just the silly stuff you can do exploring by your self. The racing really is where it's at, as long as you've (once again) removed all driving aids and set manual transmission. Many early races are like cherry-picking; easy to win, easy money, but cheesy accomplishment. However, most are real challenges especially the circumnavigate-Oahu types. If you like driving recklessly in traffic on a beautiful tropical setting, you'll relish the races in Test Drive Unl.