I set people up with standing desks & treadmills. Amazon is my favorite place to get the motorized desks (ex. FlexiSpot), as you can typically get them for under $300. The price goes up from there depending on the size & features you want (ex. a monitor riser shelf). For monitors, I like to go with Ergotron arms, so you can strap them onto the desk securely & then swivel, tilt, move, and height-adjust the screens as needed.
If you're interested in treadmills, those typically go for $300 to $400. Keep an eye on the length, width, and speeds, as there are some super skinny cheapo ones out there that are annoying to walk on lol. If you don't want a treadmill, they also make sit/stand stools. I'll typically setup a drafting chair that has a back & a ring for feet, in case you want to keep the desk standing but sit at the desk without lowering it.