I keep a Lamy Scribble ballpoint in my bag at all times - sleek, small, and perfect when it isn't suitable to use a FP. Also, sometimes I loan the Lamy in class - people seem daunted by being offered a FP, but will happily use the Scribble.
The refill in my Scribble blewback into the pen, and Lamy exchanged it for a brand new one for like $10 - 3 years out of warranty. Very good of them.