Quick answer: Snorkel or see the mantas in the morning, cross the Yellow Bridge to Ceningan's Blue Lagoon, see the seaweed farms at low tide and the mangroves at high tide, and end the day at Devil's Tears for sunset. Two or three days covers it.
Lembongan is small: you can cross it by scooter in under half an hour. What you do on a given day depends mostly on one thing: the tide. Check it each morning, then pick from this list.
Snorkel the reef
Morning boat trips leave from Jungutbatu to Mangrove Point and the reefs around Penida. Spots and safety.
Swim with manta rays
Off Nusa Penida's south coast, seen through most of the year. How it works.
Surf the reef breaks
Shipwrecks, Lacerations and Playgrounds, for confident surfers, best April to October. Surfing guide.
Watch Devil's Tears at sunset
Waves exploding up the cliffs. Keep well back. What to know.
Swim at Dream Beach or Sandy Bay
Pale sand coves on the southwest coast, when the sea is calm. Beaches ranked for swimming.
Cross the Yellow Bridge to Ceningan
The Blue Lagoon, Secret Beach and quiet roads. Ceningan guide.
See the seaweed farms at low tide
The island's oldest living, still worked by hand. The seaweed farms.
Paddle through the mangroves
At high tide, boatmen pole flat-bottomed boats through the channels in the northeast. Or take a kayak or paddleboard. The mangroves.
Explore the underground house
Gala-Gala, near Lembongan village: rooms and tunnels dug into the limestone by hand by one man, Made Byasa, between 1961 and 1976.
Sunrise at Jungutbatu
On a clear morning, Mount Agung rises across the strait behind the moored boats.
Learn to dive
Dive schools on the island start you on calm reefs before the bigger sites.
Day trip to Nusa Penida
A short boat ride across the channel to Crystal Bay and Penida's cliffs. Roads on Penida are rougher; go with a driver.

Questions
How many days do you need on Nusa Lembongan?
Two to three days covers the highlights: a snorkel or manta trip, Ceningan and the Yellow Bridge, Devil's Tears at sunset and the mangroves. Stay longer to dive or surf.
Is Nusa Lembongan worth visiting?
Yes, if you want clear water, reef, surf and a slower pace than mainland Bali. It is small, so you can see a lot without long drives.
How do you get around Nusa Lembongan?
By scooter, bicycle, electric buggy or on foot. Roads are narrow and hilly in places, so only ride a scooter if you are confident.
Can you do a day trip from Nusa Lembongan to Nusa Penida?
Yes. Local boats cross the channel in a few minutes. Penida's roads are rough, so most people go with a driver.
Getting there
When to go