1) Chill Touch, as I mentioned before
2) Parry could be a frustrating creature ability. In my experience, players frequently reserve bardic inspiration for turning a miss into a hit. Parry can turn it back into a miss, so you can feel extra special about being Mr. Poopypants to your bards.
3) Split the party. I've always wanted to do two running combats with two groups of PCs racing each other to the finish line. You're in a rare position because you can split the party exactly in two, class wise. Archetype specializations might be cause for some consideration, but it could be a spectacular contest, regardless.
4) Pitting the party members against themselves in a more general sense could be fruitful, too, though I'm not sure if there's room in the scope of the one shot to do that in a fun way. Something to think about, anyway.
5) Trauma ward: some form of extended skill challenge involving the use of Wisdom (medicine) along with other possible skills to stabilize a large group of survivors from a natural disaster. Failures result in NPC death. PCs can use their magical healing skills to negate a failure, but there are enough wounded to exhaust their resources if they don't succeed by conventional means most of the time. Also, some bad guys show up at the end.