A sprint is your classic HIIT workout, and you just can't get that level of all-out crazy work on a treadmill safely. Though I suppose if you can, you should really ease into things? I also just don't see any reason anyone would want to be on a treadmill or elliptical in summertime.
An example mini-workout, crossfit-style, I did AFTER weight training last week: 10 box jumps and 15 kettlebell swings, 5 rounds for time. Took me about 6 minutes and I spent a minute on the floor afterward. I might add 10 pushups to that sequence next time, that would stretch it to about 10 minutes. Another one I like is a 2k row as fast as possible on the C2 rower.