You could get the 50mm f1.8, 28mm f2.8, and 135mm but those are booring.
Try these three canon FD lenses for not too much:
#1: 50mm f1.4 - an abolute must have. Better than the f1.8, 50mm f1.2*, and 55mm f1.2. *Note the 50mm f1.2 FDn non-L version is pretty poor.
#2: 24mm f2.8 - A little bit wider than 28mm and very sharp.
#3: 70-210mm f4 - More versitile that the 135mm, surprisingly sharp.