OK, then you can probably do what I was going to submit as my trick.
From a cross-eyed position move one of your eyes around all over while keeping the other eye crossed. It’s easiest moving it just side to side but if I want to make my eyes really achy I can move it around a little up and down.
I was better at it in middle school, now I don’t practice enough.
I can also mirror or upside down handwrite.
I’m pretty good at impressions of singers. I can do Antony Hegarty, Bob Dylan, Tom Waits, Lou Reed, Gord Downie, and a few others I can’t remember at the moment.
I can stick my hand through the holes in a chain link fence. Comes in handy when you are playing catch and the ball is thrown over into the next yard. My hands are quite bendy.
I can do that too. Pretty soon I got to where I could do it with a stack of poker chips 2 or 3 inches high. Slightly missed once. It was like someone fired a shotgun loaded with poker chips across the room. 
Also, re the OP, I can blow bubbles (1/4 inch or so) with my saliva. Learned it from my 9th grade woodwork teacher.
Well, you’d still have to displace the hot tub water by exhaling into it, which I suspect is actually more difficult than just drinking it. You see, the bottle is filled with air when completed.
I can do the tongue cloverleaf thing, touch my nose with my tongue, twiddle my thumbs in both directions simultaneously, and move my eyes independently - though that one does cause achy muscles! I can also write with both hands simultaneously - but whatever I write with my left hand is written backwards.
At age 40, I can still do a backbend, much to my daughter’s amazement! (Even I have to admit that it looks pretty funny now that I’m early in my third trimester!)
And I can stick out my tongue and touch my ear!
:dubious: <- But I can’t do this. My eyebrows don’t move independently.
Me, too! Did you also learn the song ‘50 Nifty United States’ in 6th grade?
And I can also tie a cherry stem into a knot in my mouth. My kids automatically hand me their maraschino cherries when they get a kid’s cocktail.
You know how people put their finger into their mouth and flick it out to make a pop? I can do that.
… With my ear.
I also had a septum piercing at one point which has never healed over, so I can thread paperclips through my nose. I look pretty “vanilla”, unexpected holes-wise (Ha!), so this seems to surprise.
Also;
I currently crack my ankles, elbows, hips, back, neck, toes and fingers each in three places, a place in my foot where I’m not even sure I have a joint, and knees, jaw and wrists. We should hang out. And disgust people.
I taught myself to do that during history class in high school. I’m always surprised that once I learned I never really forget it. I can still do it.
I tought my Mom this one once. We were out and about once and she was practicing, and someone passing by made some comment to us like she thought my Mom had a mental problem.
My dad can do that. I’ve tried to learn it, but I just can’t quite get it. I think my hands are too small.
I can bend my thumb backwards and push the joint forward. It looks like I’m popping it completely out of the socket, but really I’m just pushing it way far forward.
You know how those “random trivia” chain emails always include the thing about how you can’t kiss your own elbow? I used to know a woman who could. She was actually surprised to learn that other people can’t. Her shoulders would bend in ways that human shoulders were not meant to bend.
Funny how people automatically add that extra step 
Knuckle, toe, ankle popping.
Ear wiggling, together & independently.
Eyebrow raising.
Bend my toes backward and touch my feet.
Freethrow shooting.
Parlor trick:
“Choose a three digit number. Write it down. Pass it to the person on your right.”
“Copy the number to the right of itself, you’ll get a 6-digit number with a repeating sequence, yes…? Now pass it to your right.”
“Divide the number by 7. What? Yes, it’s divisible by 7. Trust me!
See?
Now, pass it to your right.”
“Divide the number by 11.
Yes… it is
See?
Now, pass it to your right”
“Divide the number by 13.
Not many numbers are divisible by 13, no… but I can just divine that this one is…!
… See? I told you it is!
Pass it back to the first person.”
“This is your original number… right?”
Yes! I taught myself this many, many years ago, and every few years I check to see if it’s still there. It amazes me that it is.
My grandma is 94 and can still recite the alphabet backwards. I can do it, but have to pause once in a while to make sure I’m still on track. Grandma can get through it in about 6 seconds.
Okay, I made a video of my eyebrow “wave”.Here
I, too, can cross either eye, but I’ll have to work on moving the other one up and down. Whee!
I got another one. I’m really good at flinging rubber bands. Back in college my roommate and I made a rubber band ball (with IIRC about 10,000 rubber bands on it) which means there was always an abundance of them around. So we’d sit on our respective couches each with a bag of rubber bands, put up small targets on the TV (items that were about an inch cubed) and spend all night getting high and shooting rubber bands. This was about 8 or 9 years ago and we’re both still pretty good.
I can still hit, say, a light switch or door knob from across the room without much of a problem.
The strange thing for me is my cadence. It comes out in weird groupings. It must be how I memorized it.
zyx wvut srq pon mlk jih gfedbca
and I always, ALWAYS get tripped up at hgf that’s the only part I have to slow down and really think through it, the rest just spills out of me. I have to say it’s nice to be able to do that, looking things up (like in a dictionary or phone book) is twice as fast when you can flip backwards or forwards.
Wow. I came in here to say I can touch my nose with my tongue, but you win.
Do you have TMJ like I do?