The Highlands of Iceland offer lots of beautiful sights, hiking trails, geothermal bathing and sightseeing. Before you plan your trip to the Highlands, you should be aware that the roads in this region are usually closed during the wintertime and are not passable for small cars, even in the summer. The Highlands are completely rid of populated areas, which means that there are hardly any shops, gas stations or other services you might need during your trip.
So my advice is to plan your journey carefully and do as much research as you can. Browsing through this selection of tours and articles is a great place to start!