Oral sex is best suited for public encounters because one can try to be on
the lookout while the other has a happy mouthful, it can be performed relatively
quickly, and a minimum of clothing is shed. Mutual masturbation is a discreet
and delightfully dirty way to have public sex, with no removal of clothing
necessary and simultaneous pleasure for both of you. Vaginal intercourse is next
on the list of easily done deeds in public spaces, because skirts and trousers
fit genitals together somewhat discreetly, and it's possible to appear as though
you are standing very closely, when in fact you are engaging in penetration.
Anal sex is last on the list because it requires lots of extra lubrication, may
not be a quick process, and if you are both wearing pants, will be pretty
obvious to anyone who discovers you.