What to eat???? What to drink??? I suppose it depends on one's attitude to the do and the people involved; for those who are opposed to the whole shebang then there are recipes for
Republican Pie here, washed down with a nice cup of sour grape wine no doubt.
To celebrate; well fizz is always good and there are some excellent cheap Cavas for those on a reduced budget, but some find fizz makes them thirsty, my mother for instance; she was attending a rather posh wedding with the reception held in the garden of one of the big houses in the village, champagne flowed like wine (well it would wouldn't it?) delivered to the guests by the staff carrying trays of glasses and nibbles. So many guests that the gardener had been pressed into service to help serve. His wife used to do a bit of cleaning for my Mum so, as her thirst increased, she whispered to him "Harry, this champagne makes me terribly thirsty, could you find me a drop of beer?" He whisked off. Of course she meant a drop of beer in a champagne glass, nice and subtle like. A couple of minutes later Harry was seen weaving his way through the throng with a pint glass of beer proudly carried, all on its own, on a silver salver. "Ah!" said a rather proper gent in top hat and tails, "Beer!" as he made a snatch on the pint. "Can't have that, that's Audrey's" said Harry who continued on to deliver it to my mother who was standing there hoping the ground would open and swallow her up.
Back to the point, if you're watching it on the telly you want finger food, easily prepared in advance, and not messy, try to make each bit bite size, too many of the commercial products need two or three bites which creates mess. I know it means a lot more work but it's worth it in the end. Not too salty/savoury as the concomitant drinking will mean lots of trips to the loo and you wouldn't want to miss anything.
For a street party then any of the usual contributions are welcome, but once again try to make it finger food, everyone sitting round a table with knives and forks can create a sudden hush and break up the party spirit as well as meaning there can be a jostle for the favoured bits leaving masses of rice salad, pasta salad etc. with no meat to go with it. Big jugs of Sangria or Spritzer to wash it down will mean less drunkeness. Try to get a polypin of decent, relatively low alcohol, ordinary bitter, it can be served up into smaller glasses from big jugs and it's amazing how well it goes down in such situations.
For a 'civilised' lunch then a scout round the internet might find some of the happy couples favourite dishes but I'd go for Coronation Chicken and salad washed down with Gewurztraminer.
But what is very important is that you go for recipes that you're happy with and that everyone will enjoy, same goes for the drink, no point in sitting there feeling miserable 'cos the food/drink aren't to your taste.
Do you mean Gastronomy