Skip Seminyak, tbh. Way too crowded, and can't walk 5 seconds without a billion taxis hooting at you. Not much to do there anyway.
Ubud is nice, less people than Seminyak and some great restaurants (try Bridges or Naughty Nuri's (for great ribs)) - Monkey Forest, Rice Terraces, Elephant Cave Temple, few waterfalls etc.
Uluwatu Sea Temple is pretty cool, but make sure to go early as it gets very busy with tourists.
Another thing, most of the temples are the same, so rather go to 1 or 2 nice ones and don't bother with more than that. Be careful of accidentally stepping on or kicking over people's offerings that they put in front of their houses, restaurants or shops. They can get pretty upset. They put the stupid things everywhere. And don't buy a sarong. If you need one to go into a temple they'll provide one for free. People try to sell them to you outside the temples usually, so just tell them to take a hike.
Sanur has a pretty decent beach for lazing about, with some great restaurants along the beach front.
If you've got time, snorkeling can be great there - check out the Gili Islands day tours.
To avoid Bali belly just drink bottled water. Most hotels/resorts will provide it for free in the mini fridge in your room, and also restock it daily. Usually have to buy it at restaurants, though.
Make sure to keep your aircon running in your room - it's usually running all the time anyway. Be prepared for pretty intense humidity, too. Temperatures aren't so bad, but the humidity makes it feel like an oven.
There's free WiFi everywhere, too. Restaurants, hotels, shops, cafés etc., so don't bother with roaming or getting a pre-paid sim card.
Also, SCOOTERS EVERYWHERE! You can rent one for a day for something like 650k Rupiah, which is ~R670. Not a bad way to get around if you're up for it - their roads are a bit chaotic. Otherwise you can get a driver for around 8 hours for roughly the same price or a little cheaper. If you're in a group it makes sense as you split the cost, and the drivers are quite knowledgeable and can help you get around easy. I've got a good driver's details if you'd like - we used him recently, lives in Denpasar.
