Timanfaya was probably one of the best things I did when I visited Lanzarote. The landscape around it looks amazing, resembling the landscape of The Moon, or Mars. If you go on one of the coach tours, you can see things such as the green lake, and the jagged coastlines. The best view though, in my opinion, was on top of Timanfaya. It's almost surreal.
They show you some cool experiments on the mountain, to show that there is still volcanic activity. Such as pouring water in a hole, to create a rush of fire or steam. There's also a pit near the restaurant, where they roast chickens above the core of the volcano, roasting them.
If you're hungry or thirsty, they have a restaurant and bar/cafe, where you can buy a snack, or a meal. They also have a gift shop. When I was there, I bought a ceramic tile of the Timanfaya mountain, with the El Diablo icon to the side. I proudly hang it on my wall, as a reminder of my trip.
If you have the option to go by coach, I'd highly recommend it. They take you down on a road through the national park, around the volcanoes and the landscape. They also play an audio documentary in your language (in my case, they played it in English and German). It tells you about the history of Timanfaya, it's eruption, and all sorts of interesting information to accompany the seemingly barren landscape around you.