A few that stand out from my competitive days:
Countdown 100's: start on an easy interval and take one second off for every repeat. Depending on how much volume you want to do start about 30 seconds above you max. i.e. We would start on 1:30 (1:29, 1:28, etc). I "won" a couple of times on my college team by missing the 55 second repeat, meaning that last 1000 yards were under 10:00.
4-5 x 400 IM challenge interval, 4x100 FR strong. My intervals in college for this were 4:15 and 1:15 (yards), also did this set LCM at 4:45/1:25. This could easily be scaled by going 200's and 50's.
2x7x400 Descend Odds, Smooth Evens Also scalable to 100's or 200's
Round 1:
1- Free
2- IM
3 - 300 FR/100 BK
4 - IM
5 -100 FR/300 BK
6- IM
7 - BK
Round 2
1- IM
2- FR
3- IM
4- FR/BK
5-IM
6-BK
7-IM
Rounds of 350's and 400's on the same interval. i.e. 6x{350, 400} on 4:00. Harder to scale at shorter distances, but could be done as 150's and 200's.
Fridays were always called Vo2 Max days (whether that's truly what the set does is another matter, but very hard none-the-less). 5x[3x50, 150] 50's are on a very hard interval and are fast - i.e. <10 seconds rest, 150's are easy. At my peak, we'd go the 50's on :35 and 150's on 3:00. This is a low volume way to get into the pain cave quickly if you do it right.
I wrote this, you should read it:
https://www.slowtwitch.com/...n_Swimming_6700.html