Steven, I cross-trained through a recent injury, mainly I did the elliptical and water-running. I think that both of these helped me maintain my fitness and more importantly, kept my "schedule" similar to when I am running. It is much harder to come back to working out once you fill up the time with other things, since you have to come back both fitness-wise and "schedule-wise" ;-).
Of the two exercises, water-running was the best in terms of calories expended and most like "running". I bought a pair of special shoes that give more resistance (I seriously believe these helped me get the most out of my workouts and were worth every penny: http://www.aqxsports.com/
). If you do this, the only other things you need to invest in is a waterproof holder for an mp3/ipod -- water running (for 1 - 2 hours) is seriously boring otherwise ;-) .... I used: http://www.google.com/products/catalog?q=waterproof+mp3+case&cid=17076108413500681058&os=reviews
(note waterproof headphones are not included). A little investment will go a long way! I used a flotation belt, they have them at the local YMCA here, you might need to invest in one of those if your gym does not have them - they are under $30 :http://www.swimoutlet.com/ProductBrand.asp?Brand=1006&Click=1719049&gclid=CNrSuKjfnKYCFQEMbAodLnv6pg. You need to have a deep end pool (at least 6 feet) - the actual motion is more like a stepping motion ... I am linking a couple of articles that I used:http://www.runningtimes.com/Print.aspx?articleID=6032http://www.jumpusa.com/aqx_aquatic_training_shoes.html
(the aqx_aquatic_training website has a lot of cool information).