Hi @labellalingua and welcome to SteemIt. I just got back from Iceland a couple months ago...it is an amazing place! With just a few days, and during the winter there, you will probably have to stay pretty close to Reyk. However, you should venture out a bit as Reyk is pretty small and there isn't much nature stuff to do there. I would suggest the golden circle tour. Depending on how speedy you are, you could do it in just a couple days, stopping off for some beautiful sights along the way. Here is a blog that details what you would see during this time.
https://iheartreykjavik.net/2016/06/what-is-the-best-golden-circle-tour-in-iceland/