In what activity are the participants engaging? That can make a difference, depending on what muscles they’re using the most.
If you’re short on time, rather than skipping any stretches, my first suggestion would be to combine two stretches at once. For example, you can stretch chest / back / shoulders while standing in a calf stretch or a lunge. Then, you don’t have to reduce the amount of time in any stretch or the number of stretches, you’re just doing it more efficiently by doubling up.
But if push came to shove and a client told me they would only do three stretches, period, in the upper body I’d do something to open up the chest and retract the chin. In the lower body, I’d do a calf stretch (on each side), and a lunge to get at both the quads and the hip flexors. With most of my clients spending hours a day at the computer, the chest is tight and closed, and the hip flexors are short and tight so this is sneaking a bit of corrective stretch into their lives. And our feet take a beating all day in and out of class, hence the calf stretches.