No one has been nasty. This is AIBU! Why on earth did you post in here if you just wanted ideas?
Also why say in the same post that most people are being nasty, and then say most people agree with you about the sweets. Which are we then? AIBU is dying to know obviously.
For what it is worth then ... no just water isn't good enough, you do need some variety. As it is a party I like to get the posh squash in the bottles - the organic ones at our local health/ethical shop. They're lush, and it is meant to be a treat. Watered down fruit juice is great. I've not done fizzy drinks myself, and I haven't seen them at a party here. But people do 'champagne' which is fizzy water mixed with fruit juice.
I don't like processed food either. I think the BBQ sounds great (you don't say that in your OP it sounds like you're just expecting to give them cake). Personally I would give them jelly too, and ice cream.
The cake is a big thing around here though, and I've been to plenty of parties where children leave with cake rather than a party bag, or cake and a small toy. I love that.
Also, I have a very simple, old sewing machine that doesn't do much except straight lines, but it does make a passable small, square, drawstring bag. Always a good option if you have the time.