Draw a pipe on the screen. First pipe is short and straight. They get longer and have more curves. "Drop" a ball into the pipe. The user tilts their phone clockwise or anti-clockwise to guide the ball through the pipe. That tilting is the only control the user has. Each pipe should be short enough to create the "I nearly did that; just one more go" effect.
http://up-for-grabs.net/ has a list of projects with issues/tasks that are easy for new contributors to pick up.
I've built one using an old nokia that supports AT commands.
Pretty fun stuff and no soldering required. Next up I'll try adding a camera and send MMS messages through the mobile phone with photos when motion is detected.
Helpful links:
http://www.amazon.co.uk/Emgreat-Detecting-Pyroelectric-HC-SR...
http://www.amazon.co.uk/Pyroelectric-Infrared-Motion-Sensor-...
http://www.raspberrypi-spy.co.uk/2013/01/cheap-pir-sensors-a...
http://www.adafruit.com/products/189
https://www.modmypi.com/blog/raspberry-pi-gpio-sensing-motio...
http://raspberrypi.stackexchange.com/questions/13334/how-can...
I have been working an a side project recently. The guts of the code is very simple and done already. Getting all the surrounding framework to serve it up as a web page and do input validation is taking forever (mainly because I only get an hour or two at a time to work on it). Its all the "polish" that takes the time. So something command line based would be easier.
Flickr creative commons image search engine
Twitter bot
Quote scraper
Deal alerter for craigslist
IFTTT on a scale relevant to you
But they're still fun.
My bots have always been @reply bots, hence my confusion.
In particular, I was doing stuff like this: "For example, sending automated @replies based on keyword searches is not permitted."
I tried to make an agregation of all the people I was following, and mining their 'about' text, and right off ran out of api requests quota.