In Kuta, you can go to Sukawati which the the local's market for shopping. Stuff there are relatively cheaper than in Ubud even for the same thing. There's nightlife there like clubbing etc.
In Ubud, its rather serene. At night you can go to 'town', walk and shop around. Not much of a nightlife though. Day nite, there's many local tourist attraction like Monkey Forest.. Bersakih.
In addition, you can always take a cab to Jimbaran for sunset and seafood dinner. Also, don't miss out on the massage, spa, mani & pedi. Very cheap!