Replying to Camera dillema

with biking season coming up quick, ill be shooting alot. i currently have a completly stock canon digirebel xt with an ef-s 18- 55mm lens. im really not sure whether to get a bigger lens, a fisheye, or an external flash. either one is arguable in my mind, but probly the flash most. as ill be shooting in woods, you dont have as much light, and its alot harder to get a good focused shot on your subject, cause there are trees and leaves everywhere. if i am to get an external flash id like the option of it being either wireless, or having a cable attachment with the flash on a tripod. anyone with some insight care to give any tips on what would be best to get, and around how much one of these set ups will run me?