Integrated system ideas/projects

Hi. I am currently a Year 12 student and I need your help. Im looking for some ideas for projects which incorporate at least two systems (preferrably electrical/electronic and mechanical). It needs to be something that is not too costly either. No more than $100 Aussie dollars if possible.

Thanks

A line following robot. Or a light seeking robot. Or a maze finding robot.

Plenty of tutorials and parts lists for those robots on the web. You could try out some of the beam robots, too.