I went last devember and I stayed in hotel in kuta for 4 days and another 4 days in villa in seminyak area.
Kuta is very noisy and has alot of shops and malls. I didn't like their food most of time i used to eat pizza hut.
Seminyak is in the city center also has street with alot of cafes and restaurants
Regarding the activities: we did water rafting , elephant ride , water activities in nusa dua such as parasailing, banana boat and the best one is ocean walk. I also went to jimbaran to have a romantic dinner with sea view but unfortunately the weather was rainy and we stayed indoor and the seafood wasn't that good.
The best day was in potato head beach club it is an amazing place to spend the day
We also went for shopping in beach walk mall and discovery mall both located in kuta area