Send out invites with details of date, time (eg 11.30 - 12.30), the organisation you are supporting, and a suggested donation - eg "donation of €5+ appreciated" or whatever. You will have more people attend when they know exactly what to expect and many of them will leave a higher amount.
Prepare your table with flowers, serviettes, cups, glasses, plates, cutlery and a container clearly labelled with the name of the charity for the donations. If you have any brochures about the organisation, you can leave them lying around to give your guests more info, or prepare a little info leaflet yourself. Use disposable plates covered with d'oyleys to display the food to save on the washup. Music in the background is always nice.
Drinks - tea, coffee (cream, sugar and milk available), natural and carbonated water
Food - Small cakes (think Mr Kipling size), sliced warm apple tart with cream, fruit loaf (butter pats and jam portions), most people love warm scones.
For the weight conscious - pieces of fruit or cocktail sticks skewered with lowfat cheese & pickle or grape & pineapple or whatever takes your fancy.
If your guests don't know one another you could always give them a name sticker they can write on and wear. You will then be able to introduce new arrivals easily.
You could hold a little raffle and ask for donations of raffle prizes - you could include the usual wine/chocs plus something as simple as 'will do a week's ironing, 'will bring dog for 5 walks', 'will cook party-size lasagne/desserts' etc.