500$ to spend on camera gear?

So I have 500$ to spend on camera gear and I don't know what to do. I have two lenses, one standard 17-55 and a small tele 50-200. I own an old Nikon D40 and I'd possibly like to invest in a new camera body. I don't really have any filters except for infrared.
I'm interested in outdoor, spontaneous photography. Nothing studio.
Any tips on what I should get?

For $500, it's hard to say. I would consider various accessories, things I can think of (pick & choose):

tripod/ball head (~$250)
monopod (~$150)
camera case ('$75-$100)
battery grip/extra batteries/memory cards (~$125-$250 (brand dependent.
UV(0) filters for all lenses (~$50-$100)
CLA (cleaning/lubrication/adjustment) on the D40 (~$150)

Not sure on your prices over there. But Go for a new camera body. Maybe a Nikon D3100 or and Nikon D90 depending on your prices in the USA.

A tripod is always helpful for outdoor photography because it reduces camera shake, particularly if you're using slow shutter speeds. I'd also recomment a polarized filter. For landscape shots, you could look at getting a wide angle lens. I would do that before upgrading the camera body, unless your D40 is falling into disrepair.