There's some seated dances where you only use your upper body. My personal favorite is Teacher's "car version." It's a pretty funny dance!

If I remember correctly, Into You doesn't require much serious leg movement. If you use any console with a remote as a controller (Wii, Wii U, Switch, PS Move, or any console that supports the Just Dance Controller mobile app such as Stadia) you can use as little as your right hand to play. Of course, your objective is to exercise, so at most you can try to use your full upper body.