Oh my, lots to do, without knowing where you are staying, first night Jimbaran Beach for the fish restaurants, too many to choose from, Roma's always a good bet, it's about halfway along the beach, you can't miss them as they have very high Alang Alang roofs, get there by 5:30pm, for a few sundowners and get eating, good evening atmosphere as most of the beach restaurants have strolling bands which are a real hoot. If you have seen Air America at Gino's you'll know what I mean.
Next day get out into the countryside, my suggestion would be around Jatiluwih, absolutely stunning scenery and a very good Cafe there where you can stop for a break and look out over the terraced rice paddies. Ubud is easy to get from there so that could be your next stop, too much to see and do really, food, spa's, bars, dancing, music etc. Classic places are the Lotus and Murni's.
Second and last evening, try Seminyak, like KuDeTa, Zanzibar, etc, great bars right on the beach to watch the sun go down and good restaurants to eat. you could also try Made's Warung 1 or 2, number 1 is at Bemo corner, while number 2 is great for some people watching.
The most important rule with Bali, is be ready to change your plans at a moments notice, as you travel around there is a good chance to come across a family/village ceremony which you really should stop and be part of also when out in the countryside, be prepared to go in another direction if it looks interesting.
If you like a good empty beach for surfing, try Dreamland on the Bukit Peninsula. If your after a cheap stay try Mick's Place.