An absolute must in Iceland to visit is the glacial lake Jökulsárlón. You can explore the place on your own or opt for one of the many tours. There are boat trips on the glacial lake, but also tours that take you along the ice caves.
Many people go along the Golden Circle in Iceland, this place is quite a bit outside but it was well worth the long drive as far as I'm concerned. On your trip here you will pass the black beach of Reynisfjara. Nice to combine!
Eating gluten-free in Iceland is not very easy, but luckily there is a good restaurant on the route that offers various gluten-free options. At about half an hour after the Glacier Lagoon you will find Foss Hotel, even if you don't stay here it's a wonderful place to eat. In the restaurant you have a grand view of the rugged landscape. The staff are well informed about gluten free food and are happy to tell you which options from the menu are safe to eat. The food is not cheap, like everything in Iceland, but you can count on an excellent dinner!
If you do want to spend the night in this hotel, they can provide you with a gluten-free breakfast including gluten-free bread.
Tip: book in advance if you want to eat here on your day trip to the glacial lake.