Yummmo... afternoon tea... let's think.... (you could call it 'high tea' if you want to be really classy!)

scones with jam and cream
little sandwiches (fancy ones with no crusts! - egg and lettuce, roast beef with tomato, ham and salad)
petit fours (make or buy) (or similar - a couple of slices, cupcakes, fudge - whatever you do best)
ice creams and fairy bread for the kids
a packet or two of chips
feta, caramelised onion and cherry tomatoes on mini squares of puff pastry (or melba toasts)
gourmet party pies, sausage rolls and quiches
cheese and spinach triangles
nice teas, coffee and maybe bubbly?
and of course a yummy cake for the birthday girl! (or use the cupcakes as your cake)

avoid to keep costs down: cheese platter (cheese cost a fortune!!)
beer (maybe you don't really need at afternoon tea)
nuts
fruit (can actually be really pricey, and at the moment we don't really have the nice cheap summer fruits)

to make the day easier:
make ahead and freeze (or buy and have in freezer) quiche, party pies, sausage rolls, cheese & spinach triangles
day before - get everything like chips (stuff that's in packets and doesn't need to go in fridge) and put it on the table (or wherever you're serving it from) sitting in/on the bowls you will be serving it in, get glasses out, plates/serviettes etc - then go out with your friends and have a ball.

beware: you don't need all of this food! it's just some ideas... remember everyone will be full from lunch!

yum yum yum now I'm hungry!!

Hope this helps give you some ideas - happy planning!