Yup, glow in the dark jelly is tonic water and UV light. I was thinking of the coloured celery as well - trouble is that natural food colouring really doesn't do the job (I've tried) and not sure I want to tartrazine-up the guests! We're going to blow the candles out by making carbon dioxide (baking soda and vinegar).
Am a bit wary about drinking out of beakers - would need to buy a fresh set! Although did get some plastic "test tubes" in the supermarket, I think they sell them for people to have shots. I did buy some plastic beakers and am going to mark lines on them to look like beakers.
Sounds like I've got lots of the same plans as stealthsquiggle - am planning on diet coke and mentos, fizzy tablets in film canisters, fake snow, slime, elephant's toothpaste etc. We're going to build structures with marshmallows and (uncooked) spaghetti. I've made some "labcoats" by drawing on white t-shirts with Sharpies, am going to borrow some safety goggles from school and offer hair gel to achieve the "mad scientist" look.
I think standard party tea will be the way forward.
Party bags - I got some brown bags, have printed out hazard signs and stuck them on, and was really lucky in our local ASDA just after Christmas as they were selling off the Science Museum science putty for 50p a tin, have also bought them a glowstick each on ebay.