When in Bali, it is important to figure out which area you will keep as your base. You can stay in one of these places and travel to the other spots of interest.
A few days in the heart of Bali, and a few days near the beach.
Ubud: centrally located, more traditional, culturally vibrant, dance performances, authentic balinese cuisine, balinese massage, street shopping, temples, more homestays if you want, else resorts as well: the image of bali that is widely portrayed.
Seminyak: beachside, more cosmopolitan, lots of cafes, great for shopping, romantic beachfacing dinners, classy spas, beautiful villas and resorts, surfing, a perfect beach vacation spot. Nusa Dua could also easily be a beach option.
Lovina/Sanur: If you have more days, Lovina or Sanur could also be considered. We didnt have the time, but we would have liked to try these two quieter places as well. Lovina could also be a day trip for the dolphin tours.
Kuta: We avoided this area as it is the younger, crowded region with lots of pubs, clubs, dancing and loud music. This was not our idea of this vacation, but we have heard that it is great for those who want a dose of the party scene.
You could first stay in Ubud, and then in Seminyak, as it would be easier to get to the airport from Seminyak on your way out.
We spent 3 days in Ubud and 3 days in Seminyak and found it just perfect.