With Halloween tomorrow, it’s gotta be the classic "Satan Set."
Make sure you get in a good warmup and a short set that gets your heart rate up slightly (some build 25s are always good), then…
6 X 100 ALL OUT and from the blocks on a 6-minute sendoff (approximately) and all of them within 6 seconds of your best time.
Do an easy 150 or 200 recovery swim between each of the 100s to get your heart rate down. Have fun, and happy Halloween!