If you have not done a glacier hike yet, I can highly recommend the one at Fjallsárlón. They only take small groups and are the only company going to this glacier. We were the only ones on the glacier and had an amazing guide (Karolina).
The hike to the glacier is not super far so it is suitable for both seasoned adventurers and for people looking to try something new. I always love seeing the glaciers from up close and this was the most fun glacier hike I've done!