The restaurant at Geysir is of excellent quality but quite expensive. The advantage: a tremendous view of the thermal field. Enjoy your romantic dinner and watch Strokkur erupt every few minutes through the restaurant's picture window.
Main courses are quite pricey -- from Kr 1550 for a veggie dish to Kr 3700 for Reindeer. To save some money, go for a soup or a light dish. If you do, you should be able to get out of the restaurant for Kr 2000 or so.
Favorite Dish: I had some nice fish soup with curry. Sara tried the bacalao tempura, which can pass fairly well as a light evening meal. If you're adventurous, try the whale carpaccio.
There is a souvenir-shop next to the Geysir hotel. In this shop there is also the Geysir geocentre and a restaurant. There you can get hamburgers and the normal fast-food, but also the national Icelandic meat-broth, which is very popular here.
The Geysir hotel is next to the Geysir shop, and in that hotel there is a good restaurant, this restaurant in the Geysir shop is much less fancy. But it has an outdoor porch where you can enjoy your food and watch Strokkur erupt. So this is where we go when visiting the Geysir area.