Víti is a geothermal lake in Askja kaldera. Temperature of the lake is around 20-25 celsius which means you can take a quick dip in there and take some cool shots, but the lukewarm temperature won't allow you to stay there for too long on a cold day.
If you decide to bath there, be careful, the floor of the lake is laid with slippery clay and sharp lava stones.
It takes about 15min to reach Víti from the parking lot (which translates to hell) and the bigger lake next to it Öskjuvatn.
To get there you can take road F905 (easier with mid-sized vehicle) or road F88 (bigger river crossings) . Seek advice and check with your rental company first about insurance before driving there.