You'll need a full day to visit Beratan which is about an hour and a half north west of Ubud.
The famous temple perched over the still lake was built in the seventeenth century. Dedicated to the water goddess Danu, there are four buildings in the complex - one specifically for Danu herself and to give offerings for fertility and property.
The other three are for the Hindu trimurti gods, Shiva, Brahma and Vishnu. There is also an older buddhist stupa on the site. The iconic temple is pictured on the RP 50,000 note.
The temple is now surrounded by a family-oriented park which is popular among local tourists so try for a weekday. Bedugal, where the lake is located, is around 1500 metres above sea level so it's a little cooler here than elsewhere in Bali.
The lakeside scenery is lovely and the temple is charming. The gardens surrounding the temple are pleasant. If it's not too hot, think about taking a boat out onto the lake. There isn't a lot of tourist information - this place is about scenery and architecture.
The complex is on a site believed to have been a site of worship predating Hinduism in Bali. The lake is the second largest in Bali; it's used to be the largest until an earthquakes split it in three!
If you have time after the temple, you should get your driver to take you up the hill to see the other two lakes (Bryan and Tamblingan) from the top.
To make the most of your day trip, stops on the way to Beratan might include:
Sangeh monkey forest (calmer and easier than Ubud's forest)
Pod coffee factory (excellent little factory with tasting available)
Taman Ayun (gorgeous gardens and pretty temple, plus an ogoh-ogoh museum)
Handara Gate (popular instagram spot which is actually the entrance to a golf course)
Jatiluwih Unesco Heritage rice fields (see article here)
Leke Leke waterfall (very charming out of the way waterfall but a tricky climb down)
Botanical Gardens (Lovely area with a gorgeous tropical flora collection)
Entry to Ulun Danu Beratan is RP 40,000 on weekdays and 50,000 on weekends
Places to eat:
Rumah Gemuk (English conservatory and gardens with excellent food and a great view of Beratan Lake)
Lost Cafe (coffee and great food including home made strawberry jam)
Eat, drink Love (cosy, fun cafe on the way down to the lake)
Strawberry Stop (delightful spot with a beautiful garden, kid friendly)
Strawberry Farm and Restaurant (Great menu and you can pick your own strawberries)
Cafe Tahu Caravan (cheap and cheerful with nice surrounds)
Restaurant Ulun Danu (touristy spot in the temple complex, eat here if you're starving)
Batu Kari Kopi (Great coffee and pastries, gorgeous view) - this is in Jatiluwih