Kneeling 69
The Kneeling variation of the 69 position amps up the difficulty, though is a lot safer than its standing cousin. The man will require a fair amount of core strength and balance to hold the position, assuming he doesn't throw out his back getting into it. Although being born airborne is a lot more exciting, both partners will likely need to use both of their arms to stay in position, limiting pleasure to oral only.
Tried this tonight with GF. She is tall and slim (5'8 125) I am 6-0 and in shape so it worked perfect
Very hot and loud O on her part :)