There are many "must do's" when you are talking about Bali.
Some of them are:
1. A day trip to Ullawatu, have a massage on the beach and walk up to the temple to see the monkeys and a great view of the sunset.
2. A dat trip to Gitgit Waterfalling, stopping along the way at the Temple on the Lake, Lovina Hot Springs and the small villages along the way.
3. Take a day trip to Kintamani Volcano and on the way stop and watch a traditional Barong Dance at Batubulan, stop at rice terraces in the countryside and stop at Ubud and visit the Royal Temple.
4. Spend the day on Kuta Beach, having massages and swimming.
5. A day trip to Lembongan Island to go diving, snorkelling and seeing the locals.
6. Partying hard in the many Clubs and Bars and dining out every night.
7. Shopping to your hearts content. Ordering hand tailored clothes and shoes and the prices are bargins.
Quick Tips:
Don't drink the tap water or eat salads or any food that you feel has been washed under the tap or you'll end up with Bali Belly. That goes for brushing your teeth. ALWAYS USE BOTTLED WATER.
If you wish to escape the flocks of people who hound you on the streets to buy their wares then head to the apartment shops and markets, or better still go to the beach. These people are not allowed on or in these areas and that goes for the masseurs as well. If you want a massage on the beach you have to invite one of the masseurs down.
Always eat where you see the locals are eating, they should know where the foods good and where it's not.
Best Way To Get Around:
As we were a group of six people we found the best way to get around the island was to hire a minivan for the day (sorry there is a name for these services, but I can't recall it at the moment). These vans can be found outside most of the popular resorts and hotels.
The best thing is to negotiate a price, for the day, with the driver before you get in and start of on the days adventure. Everything is negotiable in Bali.
The taxis are pretty cheap, and if you're not going far, they are the best way to get around. This is how we go from Kuta to Jimbaran for dinner.