How much time have you got?
Vistaprint do free postcards - you could put a nice thank-you card with pictures and a personal message in each one
Try poundland for old-fasioned toys - cheap marbles, jacks - I bought 8 small errrm WTF are they called, those springs that climb down stairs? - 8 for a quid in poundland!
Poundland again for a large bags of traditional Cadbury's chocolate bars
A sachet of Horlicks?
Have a look in Ebay under "Wholesale"
Hawkins Bazaar might do something suitable - you need to look for bags of stuff.
M&S often do posh sweets which are individually wrapped, such as those Italisn almond biscuit things, which are nice
BALLOONS, got to have balloons
Ebay is good for a box of those polystyrene glider planes - they are traditional and you could get 40 for a fiver or so
Big packs of bubbles are dead cheap and everyone loves bubbles
big bags of gold choccy coins - you can split these
Search on Ebay for 'party bags' and you'll get lots of suggestions too