If I were to visit only one of the landmarks in Lanzarote, I would visit Timanfaya National Park. It is famous for its lunar landscape.
The landscape was formed by the volcano eruptions that occurred between 1730 and 1736 and it remained almost unchanged since then, because of the government protection and almost no rainfall.
The most exciting fact is that the Timanfaya Volcano is still active (it gave the name to the park itself).
You can get there easily by car following the LZ-67 route. The charge the entrance fee when you turn from LZ-67 to LZ-602 (take enough money with you as they don’t accept cards there). I recommend that you buy a discounted Bono Ticket for visiting several tourist points on Lanzarote (It is cheaper than buying each ticket separately).
Following the road, you will arrive to a parking lot next to the El Diablo Restaurant.
I recommend visiting the park in the morning so you can avoid the entrance queue (there was a queue when we were leaving, and it worked on one-out-one-in principle).
First thing after arriving there should be to get on a bus (or queue for getting on a bus — we waited just a few minutes there) that will take you for a tour around the park. It is a protected area so you can’t go there by yourself. The tour has a commentary in Spanish, English and German and the bus driver stops at various places, so you can take the photos (but only from inside the bus, no getting off).
After returning to the place where the bus tour started, you can visit the demonstration that the volcano is still active. There is a hole in the ground where the heat from the volcano gets to the surface. The park guard throws some brushwood there and after a while it ignites.
There is also another demonstration where they pour a bucket of water into another hole. After a while a geyser of steam will burst out of the hole. Pretty awesome.
Then you can visit the grill where they prepare the meat on the heat flowing from the Volcano.
The El Diablo Restaurant offers a panoramic views of the park’s volcanic craters, calderas, cones and black lava fields and you can order a meal prepared on the volcanic heat.
The grill and the Devil-shaped park logo is a work of the artist César Manrique (who designed almost everything you will see in Lanzarote).
There are various guided bus tours which take you to Timanfaya park and to other places, so you don’t have to rent a car.
There is also a possibility of taking a Camel Ride over the volcanic landscape next to the Timanfaya, so don’t forget to stop there (It is included in some of the guided tours)