From the park Visitor Centre on Rte 36 (parking 500kr), follow the path from the outlook down to the Lögberg (Law Rock), and the only standing structures in the great rift. You can also approach the waterfall Öxarárfoss from a parking area on Rte 36, and hike down into the rift valley from there. Or, come in on Rte 361 on the eastern edge of the site, and park on Rte 362 there (parking 300kr). Get mapped reference points at www.thingvellir.is.
Diving & Snorkelling
One of the most otherworldly activities in Iceland is donning a scuba mask (or snorkel) and dry suit and exploring the crystalline Silfra fissure, one of the cracks in the rift valley. There's also a rift, Davíðsgjá, out in Þingvallavatn lake, which is harder to reach. You must book ahead with a Reykjavík dive operator. People with their own equipment must have licences, dive in groups of at least two, and buy the permit (1000kr) from the visitor centre or www.thingvellir.is
Check in with park centres for lake fishing rules (some areas are off limits, and any imported equipment must be disinfected), and get a permit (2000kr per pole per day; May to mid-September) from the Information Centre. The fishing card offering fishing access to multiple locations (www.veidikortid.is) also covers part of Þingvallavatn.
In the valley on the Rte 36 approach from Reykjavík, you can go horse riding and mountain biking with Laxnes.
There is camping in Þingvellir National Park, and hotels, guesthouses and cabin accommodation around the southern part of Þingvallavatn lake.
There is a small cafe with a mini-mart at the Þingvellir Information Centre; otherwise bring your own food.
Free one-hour guided tours run most days from June to August; these were at 10am and 3pm at the time of research, but check ahead at the visitor centre for the schedule.