Shorts are your best option. Vodka and diet lemonade (no syns in the diet lemonade unlike normal lemonade). 25mls vodka and d.lemonade 2.5 syns.
I've just found a new love for gin and slimline tonic for this reason. 25mls of this is 2.5 syns, 35mls - 4 syns.
Malibu and diet coke is another one.
Try and stay away from alcopops, pints and wine (though a spritzer with soda/d.lemonade isn't as bad).
Hope that helps, but most of all enjoy.......it's only one night and we all need to have a life as well!!!