http://www.dezignstuff.com/blog/
this blog periodically puts out solid works challenges. they are more into complex parts however. i had a little fun with klein bottles recently, i find it a good exercise because it works in surfaces, 3d sketches, math, and it doesn't take too long. if you are really serious, then re-design old robots. i re-did our 2011 robot at least 5 times, and i did 2007-2010 once as well.
My best Klein bottle:
