In no particular order:
Small drop of food colouring (use a syringe), inside the wine glass above the stem, turns wine into whatever colour you fancy but does not spoil the taste.
A Met Office helium balloon with talc inside is great during the speeches, especially if you can get it over the candles on the top table, managed this once, balloon burst and speaker disappeared into a Hiroshima cloud of Johnsons & Johnsons finest.
Have a word with the Mess Manager and substitute some of the menus with ones containing made up course/wine names, you can have a real dig at people with descriptions of food aimed at them.
Radio controlled micro-cars and tanks running around the tables are good during the speeches as are the radio-controlled airships.
A smoke machine (hired from a prop company or even the local mobile disco) and lengths of hosepipe is always a winner.
Small water pistols or syringes filled with water are useful to start some inter leg rivalry during the meal.
Have fun.