Alright, so I looked at your pictures, here are my suggestions in the form of most payback for least money/effort spent:
EDIT: I read your post above, sounds like you want to keep the rack. Still consider my suggestion of integrating it (solar panel) into the roof though. As for what model scangauge to get, check to make sure your car has an OBD-2 port instead of one of those diesel truck ports. If it does have an OBD-2 port, I say go for the Scangauge II.
Remove roof rack (free): It looks like you have a solar panel up there, but trust me, that ain't worth the increased aero drag the roof rack is causing. Maybe try to integrate the panel into the roof?
Weight reduction, bro (free): From your pictures, I would say that most of the weight you put in your van if fulfilling some useful purpose, but in case some isn't, take that crap out!
Air dam ($5): Looks like you already have an air dam, but it can be improved, maybe lengthened. If it was flush with the outer edge of the bumper instead of recessed that would be best for aero.
Grill block/bumper sealing ($5): seal up that huge gap in between the bumper and grill, and start blocking the grill. Make sure to watch transmission temperatures. From what I have read on my accord forum, the bottom part of the radiator cools the transmission fluid, so try to trace the lines and find out if your radiator cools the transmission. Then block the area that doesn't.
Rear wheel side skirts ($5): Cover up the wheel wells with coroplast, consider leaving a gap at the back to let air out instead of parachuting.
Mirror delete or replace with smaller mirrors ($0-20): Depending on what you do, this could be free, or just cost a little bit. Those mirrors aren't too big, but they are bigger than most car mirrors.
Undertray ($40): Depending on how much you want to cover up, this could take a lot of time and money, or very little. My suggestion if you don't want to do a full undertray is just make a front and rear undertray. Those give the most fuel economy benefit along with stability improvements as well.
Remove the step side skirts (free): Those skirts are acting as mud flaps and catching air, however, at the back it does have a deflector for the rear wheel. I don't know how much this would really improve gas mileage, but it could. If you do remove these, make sure to add a deflector back for the rear wheels.
Delete external antennae (free): Free, but least benefit for the work involved.