Bali is small enough that you can explore with Ubud as your base.

That being said, the other parts of the island are also beautiful. Watching the dolphins at sunrise in Lovina is spectacular, Amed is beautiful and quiet on the north coast, and the south with Uluwatu is amazing as well.

Ubud is centrally located and a good place to be. Great food. Nice vibe. It's getting too popular for its own good though...so be prepared to not be alone

Enjoy!