I’ve got a few examples off the top of my head. The aforementioned Surprise Vampire using the “mystery bat” is always fun (good once per group I guess). I like rigging the item deck too, which is super simple to do.
While I haven’t done a grid or campaign, I’ve run a few sessions with branching paths, with the straight route providing fairly standard escalation and the branches usually containing a mini-boss followed by a special room or boon before tying back into the main route.
I’ve allowed “modded” heroes before as well. My younger nephew, for example, really likes using the Paladin from Soloth, but he tends to turtle rather than tank, so we let him start with a throwing axe to give him a bit of incentive to get in there and eat some thwacks. The base characters now have formalized variants, including a tank-ier Xoric, so I’ll be curious to see if he opts for him next time instead.
Another thing I’ve done (and which now has a formalized ruleset, albeit in a different way) is allowing enemies to start on top of pillars for ambushes. The new Ice Box stretch goals include Boulder traps which operate in a similar fashion (Overseer may topple them onto heroes at any time), but I had generally used skellies or zombies perched precariously atop the barriers (a good hard smack could potentially knock them off). Fun times with that.
I’m sure I’m missing a bunch of stuff (I like to introduce Antients and Wyverns in unusual ways), but I’m always looking for new ways to surprise my players, especially when they’re kicking butt.