Nusa Penida is a great place to visit when in Bali. It is nice and quiet, has some beautiful places to see such as the very famous T-rex cliff, and you can easily get around by car or scooter, before sailing back to the mainland. If you are into diving, this is a likely place to see manta rays.
Sunny Café serves great food and is just a 5 minute walk from the pier. It's a great breakfast choice if you get to Nusa Penida with an early boat, before heading of to explore the Island.