I can't even describe what a huge fan I am, of the Garda and their institution, but I'm going to try.
With 3 of my long term characters I have seen jailing variously. I have been fortunate that even as reckless as I play my characters - I have enjoyed much roleplay that was before you cross the line and need to be executed... really as a service to the setting and the server.
So my characters become that one prisoner that Garda aren't necessarily willing to kill, but is still a rather serious problem, you see...
The Garda have fined them, assigned them labor, tortured them gruesomely, in just about every single way. I think we all enjoyed it because I was engaging their characters in their element; perhaps even frustrating my captors with all my religious bad juju. In a way, we all went to town.
And yeah, lots of down time too. But if you're really out there swinging, downtime for you is noticed by others and influences their roleplay. Yes, it's flavorless for you, sure, but it can be sweet nectar in ways for other people.
Regarding the Crow Cage or Stocks suggestions:
I see escalation. PC Garda can't personally enforce against whatever Joe 20th level mage decides his moral compass tells him about this situation. Anyway, mass murder of PC Garda and their NPC support that would naturally ensue, and you'd need a DM there anyway for that whole brutal murder in public spectacle thing, it can be negative for your health.
Moreso than that, the above spectacle is not what I personally desire to be a part of in Ravenloft. It's not something I'd show some players in pen and paper that were new to the game. On paper, you could use stocks and stay true to the setting, but here? Even if you put the stocks in the Market District with a half dozen guards around it, DMs would have the new chore of enforcing in an area that doesn't run so well without them.