My favorite scent for my rig is cinnamon/pine cones. Short of picking up some mesh baggies at the dollar store and raiding my spice cabinet, the best method for accomplishing this for me has been Walmart during the holidays. Sure, you can find those bags of scented pine cones practically anywhere, but Walmart sells small bags of (approx 10-12) cinnamon sticks.. but they are scented the same as the infamous pine cones.
I usually buy two of these little bags (usually around $3-4 each) and put one in my truck immediately and store the other one in a freezer ziplock type bag and freeze it. The scent these put off lasts about 6 months so I spend under $10 a year to scent my rig (pretty good considering the costs and longevity, or lack of, on most scenting products made for vehicular use).
Oh.. and for the record, this woman LOVES the black ice scent! In some years which I haven't been able to locate a Walmart for my method above, I buy Black Ice trees and hide them in the vehicle.
Very long lasting and more than subtly noticeable.