Also

• the "peekaboo fuck you" one hand covering the other, then moving the first hand for a middle finger reveal,
• the one above, but you don't move the hand. The middle finger plays shy and peeks over the covering hand.
• the "pick a finger" while madly wriggling every finger,
• the "I've got something for you in my pocket" reveal,
• blowing your thumb to inflate the middle finger,
• like above, but you make trumpet noises to imitate playing a fanfare for the middle finger.
• the ordinary extended middle finger, but pronating the hand to insinuate erectile dysfunction in the recipient
• adjusting your glasses, this worked even better if you didn't wear glasses
• picking or wiping your nose, or cleaning the outer ear, with the middle finger.
• pointing out something in a book or other written or graphical material
• after teaching people to count in binary on their fingers, you could just refer to 4
• caressing a nipple with the middle finger. This one could be made even more inappropriate if you used an accomplish for the nipple.
• the ordinary middle finger, but moving it in a "come here" motion. Only do this if you're sure that you're not about to get attacked or can defend yourself.

Tbh I kinda feel like Bubba from Forrest Gump right now, listing all the things you can do with shrimp.