Did this one once at a restaurant. All it takes is a drink (with a straw) and a bottle of Tabasco sauce. The ability to keep a straight face is highly recommended. It may take a cohort too, If your unsuspecting person doesn't leave the table.

Step 1:

When your victim, uh I mean unsuspecting target, is away to the restroom put about 4 drops of Tabasco sauce down their straw. This is when a colleague might come in handy. if you can have them distract them long enough.

Step 2:

Sit back and act innocent. This might be the hardest step. Trying to look as if nothing is up. When they take a sip the first thing to hit their tongue is the shot of sauce. From there things might get a bit heated depending on how good of a sport your (ex-)friend is.

Step 3:

This actually works a little better than just pouring the Tabasco sauce in the drink directly because one it leaves the sauce concentrated in the straw, as long as your victim doesn't play with their straw, and two since the sauce is in the straw it tends not to be as odorous. You may have to be willing to share your drink when it's done though.