All your mess mates/work mates names go in a hat, whoever you draw out you have to buy a prezzy for, normally for a fiver or less. When I was serving some of the prezzies were hilarious as they're supposed to reflect the person's character or habits.
Fortunately I work in an environment which is heavy with ex0service although if we get a civvy's name we have to be a bit more careful.
Secret as in you don't know who got you your prezzie.
I did have one in tears. Apparently she was expecting something like perfume. She's the school's reprographics officer and a big box of gash stationary from out of the drawers in my office , was not appreciated. Made me laugh tho!