The first thing I'm going to do is steal a car and kidnap it's passenger, drive up to the desert and execute him. That was something I missed in Gta IV, those long rural area's.
and ofcourse driving there with a sanchez that was so much fun in Gta SA, driving from one urban area to the next (with nothing but countryroads between it). Missions are fun but for me Gta has mostely been about free-roaming, sometimes really violently and other times just driving around and looking at the surroundings. Can be quite fun to walk around too, bump into citizens, get into streetfights etc.
Lastly I second RPGing a cougar (or the next best thing), in RDR they terrorized me.. time to get some payback