For must-do's in Bali, I would say sunset at the temple at Uluwatu, accompanied by an exhibition of traditional Balinese dancing. Just beware the monkeys at the temple-- they are theiving little devils who will take your eyeglasses right off your face and either hold 'em for ransom (bananas for the monkeys, rupiah for the locals) IF you're lucky or just break 'em if you're not so lucky. I'm not kidding. Seriously.
I would also say a car ride through some of the little villages that specialize in various arts and crafts-- just focus on the ones you like: silverware, batik silk, painting, woodcarving, masks, whatever. I have a beautiful painting in traditional Balinese style, and I'm planning to look for another one next time. Then spend some time in Ubud, the inland village at the artistic heart of Bali. It has a number of smart stores and quite a remarkable temple right in the middle of town. If you can squeeze it in, a sighting of some river gorges and terraced rice fields would be very Balinese.
Skip Kuta. On such a short trip, the only thing worth seeing there, really, is the site of the 2002 bombing. Though a simple memorial, it's quite moving.
The other coastal towns are a bit less backpackerish that Kuta and might be worth a stroll if you have time. You might get lucky, as we did, and see a mid-day procession of townspeople in traditional dress carrying a litter with offerings to the gods.
If you're a planespotter, do what you can to get alongside the runway at Ngurah Rai. A search in airliners.net for photos taken at DPS will quickly turn up images showing you exactly how close to the big birds you can get.
Above all, do your best to get in touch with the Balinese people. They are some of the warmest and most genuinely friendly I've met, right up there with the Thais.
A search for "Bali" and "driver" should turn up info on two drivers who come very highly recommended by FTers. We plan to use one of them in May.