I have a non-chair chair recommendation: a medicine/pilates ball. You pick one of the diameter roughly appropriate to your leg length, and then inflate (this takes a long time typically) to fit. Since it has no back, and requires active balancing, you literally can't slouch. I don't buy the free core-toning that some claim, but it certainly got rid of my occasional lower back muscle spasms/cramps when I switched both home and work to a matched pair of balls.

Note that there are some products that put the ball inside some kind of frame so it stays upright, but in my opinion that defeats the purpose.