Couldn't you lair flag it to prevent item scrambling?
As for the traps, I like the minecart one. Include pressure plates with a weight setting that you deliberately closed your eyes and mashed keys to set, so you have to get around through whatever other obstacles you include to find the right combination of items to put in the cart... or can you load a cart in adventurer mode? I haven't done much in it, ran at a crab, the crab exploded, I chuckled but went back to dorf mode.
If you can't load it then see if you can preset the carts with the right weighted items (could make it easier on yourself and just limit each plate to a single combination of cart+item like a stone or something perhaps?) and check to make sure the system operates properly, then rely on the item scattering to make a mess and force you to find the right two items to get everything working to finally breach the inner sanctum.
In the process you end up setting numerous minecarts zooming around on repeating loops through different parts of the very same path you will have to work your way across to leave!
Plus include them doing things like triggering floor hatches/grates/retracting bridges/smashing bridges/floodgates/weapon traps and so forth.
You get there, it is a mess, but for the most part it is dormant, barring the smattering of normal dangers, then you go through all the effort to tidy up and get things working properly to get to the delicious candy coated prize in the center, which you can now escape with, assuming the now fully armed and operational deathtrap doesn't work as advertised, that is.
Oh, bonus fun, similar to the demon suggestion above, but campfires can trivialize that... set it up so removing the final prize releases a cave-in that brings the circus to town, who will then be free to path up into a convenient waiting room, which is also blocking the only way in or out of the fort. Try to account for demon pathing to stall them just long enough that you have a chance of hitting the exit before they do... but don't include time allowance for getting knocked over by a minecart or dropped into a pit by a suddenly opened floor hatch.