Bring spares and tools to change things out. You'll probably find one car runs better than the others, so you might want to swap things around to eliminate variables.

Use the exhaust sniffer if they have one. Print or right this info down.

Extra new plugs.

Bring a DVOM for checking AFM and O2 voltages(etc), infrared pyrometer for temps around the engine, "dial back to zero" timing light for checking total advance. Careful: Miatas fire through each wire twice and a sensitive timing light will pick the second firing up-halving your advance number.

If they don't have a large fan for the front of the car(they should), jump the fan relay. A plant squirt bottle for misting the radiator will help cool it down quick.

If it's a Dynojet or similar roller dyno, note your rear alignment specs, tire size, type of wheel, and chassis height(off the dyno and strapped to the dyno) for each car.

If it's a loaded chassis dyno, note the "ramp" time.

Have fun. Be careful you don't get addicted to the dyno. Come back and post your numbers.