I have this, no complaints other than the price https://www.amazon.com/Ergodriven-Not-Flat-Anti-Fatigue-Calculated-Must-Have/dp/B00V3TO9EK/ref=sr_1_7?dchild=1&keywords=desk+standing+pad&qid=1612580197&sr=8-7
It's goofy looking, I hide it when not using it.
My feet are all sorts of fucked from the army, I brought this one roughly 3 years ago, it has been incredible. Even allows me to massage my feet.
https://www.amazon.com/dp/B00V3TO9EK/ref=cm_sw_em_r_mt_dp_U_Z3cDEbM9ZD41G
I saw it in a Wirecutter review years ago. The contours are phenomenal. My feet move around in random positions to relieve the strain and keep them from getting bored.
The most important thing to get the most out of your standing desk is a good anti-fatigue mat. Do not cheap out on that... it's so important. Most of the cheaper ones are too thin or compress too much.
I have had this one for a few years and can't imagine working without it: https://www.amazon.com/dp/B00V3TO9EK/ref=cm_sw_em_r_mt_dp_U_Sk6eDb095RT95 Take a look at some others on Amazon to see if you like the shape/size better, but this one has some great features; it is thick enough for me as a 6'6" 380lbs guy to not smoosh it, and the ball in the center is great for keeping your feet moving, as is the big hump on the back (which I can stand on with both feet and not compress). I do calf raises with the back hump to keep stretched out throughout the day.
Also, for sitting I have a seat cushion that works well and doesn't deform too much. It's also cool- I've found other ones to get hot fast for some reason. https://www.amazon.com/dp/B014F18ZGU/ref=cm_sw_em_r_mt_dp_U_hp6eDbT3R0PPE
Finally, as u/searine said, pay very close attention to your ergonomics. Look online at diagrams of how your arms/hands/neck should be positioned. Get yourself a monitor stand (I have a triple that just clamped right on the back of the desk and is very adjustable... saved a ton of desk space too!)
Good luck.