Here’s an oldie but a goodie: (please note, for best recommended results, try at liberal arts institutions known for being fans of theater)
As seniors, my friends and I greeted freshmen students (no one asked us to). One of us held a clip board and pen. One held a tailor’s measuring tape. And one had a sharpie and a small stack of “Hello my name is _______” name tags. Here’s how the prank goes.
1) Cheerfully greet each student as if you’re an official greeting team of some sort. Clip board holder says “Friend, let’s get you a name tag”
2) Measuring tape holder proceeds forward, holding out tape-measures something random about the person (something where you don’t actually need to be up close and personal, like the length of their shin from 2 feet away, or I suppose you could try asking “please may I measure your wrist” or some such). Measuring person calls out a number. Possibly two.
3) Clip board person studiously records the number on a chart. Nods gravely/excitedly/smoothly/suspiciously/enthusiastically/whatever at Name tag person.
4) Name tag person writes down something utterly totally random in the tag blank. Like “peanut butter” or “ aerodynamic jellyfish”. With great ceremony, tag person hands tag to new student. All prankers bow, or offer waves, while saying “Pleased to meet you (name tag name), welcome to college! We hope you have a great year!” And walk away in whatever style suits you best.
Done correctly, this can happen so fast and flawlessly that parents, new students and onlookers will all be awed and confused by the time you’re out of earshot and if they see you later, will simply go “hey, it’s the welcoming committee!” and laugh. And it’s so much fun.