One of the most beautiful sights in Bali is the Tegallalang Rice Terraces. It is around 1/2 hour drive to the north of Ubud, and is well worth the trip.
The Balinese practice a traditional system of irrigating the rice fields called "subak", where everyone share the precious water resource which flows through all the fields. Due to the hilly terrains of Bali - the rice fields had to be terraced on the slopes. Coupled with the natural scenic beauty of the valleys in Ubud, these rice terraces are now a Unesco World Heritage Site.
As is usual in many attractions in Bali, you will arrive at a spot where there are many shops and makeshift stalls touting souvenirs to the tourists. Here there are also many cafes and simple tea houses built on the terraces, offering refreshments while you enjoy the views. Pick one with the best views and you will be very happy.
While rice terraces are quite a common sight in Bali, the ones at Tegallalang are truly spectacular. If you come during different cycles of the rice growing season, you will see quite different views. They say the fields are most beautiful at the early stages when you can see the water - but I find the sight of the ripening rice grains quite mesmerizing.