We went to Iceland to celebrate the end of the world on december 21st. Quite nonsensical, but why not take the opportunity to have a bit of fun?
We decided to go with Hotel Laxnes in part becasue we liked the look/atmosphere of it, but also because we wanted to celebrate "the end of the world" with a glass of wine on top of a mountain (it seemed like a sensible thing to do), and there are plenty of those available within walking distance of the hotel. Allegedly a bunch of them even have hiking trails, making them a lot more accessible.
There is a bus stop right outside the hotel that takes you to central Reykjavik in less than half an hour, though it should be noted that there are no night buses. There's a mini-mall with a small supermarket, a pharmacy and an off-license, just across the street, and several places to eat in the nearby area, although I'd recommend doing lunch and dinner in Reykjavik.
The hotel breakfast was really quite good. Not the biggest spread I've seen, but there's enough variation for a week's stay. There's no restaurant in the hotel, but there's a bar that's staffed in the evenings, and on one night they did serve a traditional icelandic meat soup which was well worth trying!
If you wanna take a day tour, there is pick up and drop off right at the hotel.
All in all, we had a great time, the staff is very helpful and friendly and it's a great area to stay in. Not a lot to do or see in Mosfellsbaer, but getting to Reykjavik is really convenient.