Tropos 32

Osprey's Kickstand backpacks tend to have something like this. Tropos (32L) & Parsec (26L) both are ones I've used and appreciated for this.

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A bit larger but Osprey Parsec (26L) has the same internal kickstand as the Osprey Tropos (32L). Most backpacks that can stand up on their own only do so when 70-80% fully packed, those two Osprey bags will stand up almost empty.