You could always just stick with the 5X5 so that on the weeks you only have three days it's the days you're supposed to be lifting anyway. Then, on the weeks when you happen to have an extra freee day, maybe you could do some general fitness type stuff.
Personally, I think running stadiums are fantastic, though it does suck nuts. Jumping rope, jogging, maybe even sled pulling might be a good idea to keep you active while still not interfering with your program. What else is there? Maybe you could swim. That's a pretty low impact, high results workout. I don't know if you're gay or not, but you could always pick up a game or four of basketball if you are.

(that was a joke)
Really, anything physical would work. I think, anyway. See, to me, this makes more sense than trying to time out the DFHT on a weird schedule. Just my thoughts on the matter.