The Dentist is a good place to start:
1. He can drill tiny holes in the bottom of wine glasses - causing subtle leakage. However, last time I tried this several very expensive drill bits broke before we eventually 'fixed' one glass - the owner of which never even noticed any leakage!!
2. He can also issue anaesthetic gel, which if smeared subtley around the rim of your friend's glass will cause much merriment when his lips/mouth start to go numb shortly afterwards, or at the very least resulting in a very dry mouth and much licking of lips - especially recommended if your friend is giving a speech!!
Other classics -
1. Flour buried inside the napkin.
2. Bodge tape the chair to the table legs.
3. Use invisible thread to tie cutlery to the chair.
4. 'Interesting' pictures taped under the placemat.
5. Mobile phone with loud ring tone 'hidden' somewhere in the room, called by another mobile at a convenient time.
Enjoy
