Personally, I wouldn't do anything between even moderately heavy sets of anything.
Light sets probably wouldn't be an issue.
I guess something super easy like pull aparts would be ok, but I rely a ton on my upper back to keep me nice and tight during squats. I find pull ups really quite hard, so if I did them between squat sets there's a fair chance I'd start falling forward because my upper and middle back would be tired from pull ups.
For pump work, I have found going press/row is good. But that's about it.