Just wonnering which one to do. The camera so far is my fave. :)

camera trigger is my fave too.
Other ideas - I want to hack a moving toy so that if you shine a light on it it tries to move out of the light, but then if no light is shone on it for a while I want it to move towards the brightest light source, until its hide reflex is triggered again.
Or something similar with sound, if it detects a loud noise it tries to go away from the noise, yet if its somewhere quiet it will eventually start making a noise which gets louder until it triggers itself to hide again.
Bearing in mind I'm a complete beginner....

I'd also like to make a plant into a theramin, any suggestions greatly appreciated. I saw the 'twittering plant' on the 40 arduino projects link above, that lets you know when it needs watering, but that's not really what I'm after. I want to make the plant into a musical instrument.

Making something light sensitive is pretty straight forward. I'll do that in the beginners workshop.
To make something sensitive to light in a direction, will need more than one sensor. Think of a light sensor as something even simpler than an amoeba, it just knows if there is light falling on it. With more than one sensor, pointed in different directions, a controller, like an Arduino can decide where the brightest or dimmest light may be.

I'm not sure how you want to 'play' your musical plant.

Simple techniques for 'measuring' a plant usually try to measure the plants electrical 'resistance' (which is relatively straightforward).

I haven't made it yet, but there is a 'proper' Theremin add-on for an Arduino described at: http://interface.khm.de/index.php/lab/experiments/theremin-as-a-capacitive-sensing-device/
It really does use the electrical capacitance of the body, and not resistance, so that may be a place to experiment. I could imagine the plant, and the pot, changes as moisture changes, but it should also change as people approach it.

I use a nice circuit for detecting sound.

I extracted the circuit from this: http://www.picocricket.com/picoboard.html
The schematic is at: http://s3.amazonaws.com/jef.mindtouch.com/10033948/1160/0?AWSAccessKeyId=1TDEJCXAPFCDHW56MSG2&Signature=eY7oLCXboVlhW3%2bYvsZday57pwc%3d&Expires=1257508275

The useful trick it does is to 'hold' a loud noises (like a clap) for longer to make it easy to detect. It is pretty low-cost (well under £2) including microphone. I have a parts list somewhere, and could post that if it's useful.

For anyone with a Canon camera, I recommend you have a look at http://chdk.wikia.com/wiki/CHDK

It is an Open Source project to adds lots of facilities to many Canon camera models (camera's listed on the front page).
For those of a nervous disposition, you can add it by just put it on the memory card.

As well as a remote trigger, it supports 'bracketing' to improve your chances to get a good picture, and can even run a script on the camera to do even more complex things (I just use it for 'zebra' and bracketing so please don't ask me to write a script :-).
GB

While I'm not a massive fan of trainsets per se, the idea of hooking up a set to arduino controls seems like a really fun idea to me, perhaps even to hook the arduino up to the net (usb/ethshield). Put the controls up next to a ustream of the train online, and let the work play with it ;)

world*

My interest in attending is as a screenwriter who wants to write about some geeky kids for one project and a geek kidnapper/serial killer for another so I doubt if I'll be getting that hands on though I did do a degree in CDT education so I'm no stranger to breadboards and the like!

I will be looking for volunteers to help make things for the geek kidnapper/serial killer project i.e. someone good with remote control but I'm open to your ideas too.

Model Trains - Are you thinking of using n Arduino/Freeduino to control the power to a single model train, or using an Arduino/Freeduino to do DCC (Digital Command Control) and use model trains that understand it, or something else?

had an idea for an IR project. a quasar laser set. each person would fire a different code. when each person is hit a light/sound goes off and the persons code is stored in eprom, at the end of the day each persons logs are uploaded and score table created. It may need some directional control for the leds but has very few parts.