Consider renting a 4x4 vehicle for the duration of the trip as many roads require it. Gas prices in Iceland are high, averaging $6-$8 a gallon. Many tours include transportation from your accommodation.
Try visiting Iceland in shoulder seasons for lower crowds and better deals. Consider grouping activities by region to minimize driving times. Add extra days for more in-depth exploration of particular areas. Don't forget rain jackets and waterproof hiking boots. Lastly, it's considered rude to skip the hot dogs at Baejarins Beztu Pylsur in Reykjavik!