Hey this old chestnut! 
To start with, I really enjoy doing party bags and DC enjoy planning them with me. I don't usually spend a lot on them, maybe around £1.50 each, or sometimes I incorporate the main party bag item into the party as an activity or similar.
But this year DD is having a party at the climbing wall and I'm not sure party bags are appropriate for 10 year olds (well most of them will still be 9). So I'm not sure what to do. I'm waiting to hear if there's somewhere to do food, so at this stage I don't even know if I'm going to be feeding the guests, so would like to at least send them home with something to make up for the empty stomach. 
Any suggestions please? There will be a mix of girls and boys. Thanks.