If you are staying on the Kihei side;
My favorite beaches...
Kamaole 2-3, Keawakapu, and Big Beach. The Kam beaches are good for walking and swimming. Parts of Keawakapu are good for snorkeling as long as you get there early before the wind picks up.
If you are eating out some of my favorites are...
If you want good cheap fish and chips try Eskimo Candy. It is off the beaten path and you get a ton of food for your money. You get 4 pieces of ono with a huge mound of fries and cole slaw for $6.95. Kihei Cafe is a yummy cheap place for breakfast. If nothing else stop in and pick up a cinnamon role.... yum yum.
For the intermediate price range, sansei is the best sushi! The have great early bird and late night specials. They do have a web site that lists all the specials, so google them.
If you want an expensive night out, try DUO at The Four Seasons. It is a newer steak and seafood restaurant and is amazing!! (and for all you locals that may read this they do offer a buy one get one free kamaaina discount).
As far as activities go...
Taking a boat over to Lanai or Molokai for the day are both fun. Trips to Lanai are quicker and there are often stops along the way for snorkeling. The boat to Molokai is a 2 hour each way VERY rocky ferry. If you don't get sea sick, going over there for the day, renting a car and exploring is an amazing experience. Molokai is unlike any of the other islands. It is very underdeveloped and has so much beauty. I am not a big fan of molokini (but a lot of people are) as it is usually very crowded. They pack A LOT of boats into that little crater. I have found some awesome places to snorkel that could be on par with Molokini, like Honolua Bay for example.
The road to Hana and Haleakala are both amazing. They are both LONG but beautiful drives that allow you to view all the diverse micro climates and nature Maui has to offer. Sunrise and sunset are both amazing atop Haleakala.
One huge word of advice though... DO NOT DO A BIKE TOUR DOWN HALEAKALA!!!!! 5 years ago (before I moved here), while on vacation, I did that bike tour with a friend and her family. Unfortunately the company (Mountain Riders) was disorganized and unsafe and my friend's dad ended up losing control of his bike, went over a cliff and was killed instantly. We were going way too fast as they were trying to rush us down the hill. It was the worst thing I have ever experienced....
So sorry to end on a bad note... But hopefully that gave you some good ideas!!
If you have anymore questions let me know!!