25 or 50 m sprints in a 50 m pool. In a 25 m pool it's hard to do 50 m sprints.

Can do graduated 100m eg 25 m slow, 25 m moderate, 25 m firm and 25 m all out. I like this one for learning your gears. If you can't really explode in the last 25 you've swum too hard in your lower gears. I was told I'm meant to feel like a coil ready to spring out for that last 25 m and it feels good

Mix up the strokes if you know them.

A pair of short fins is good to help with interval work in a 50 m pool - gives you just a little more power and lets you sprint the butterfly all out for 50 m which is hard work and difficult for non-accomplished swimmers to do well without fins.

Recovery could be:
A) A set time eg 15 seconds
B) A slow swim for a set distance eg 100m before going again, or
C) a rest at the end of the pool on a timed cycle eg 2 mins to do a 25 m sprint with 75 m slow - then might have 15 secs break before your next sprint.

A few hundred m warm up and cool down is also good. backstroke makes for a really nice stretch in your cool down