Well, I don't want to spoil all the fun, but I may give some hints.
Since the source string consists of alphanumerical characters only, it is quite obvious what encoding we are talking about (well, at least to web developers it should be). There is one little catch, though. Decoding the string as-is won't work very well. But maybe if you try some other transformation (which you should be able to find out easily) first...
additional hint