Depends a lot on what role you're looking to fill.
There's very little meaningful strategy/business work that can be moved to a new hire. Most very young companies still have terrible development practices, which makes it hard to quickly plug in a new technical hire.
I think there are three possible ways it might work.
1) You're willing to act as an executive assistant, doing whatever crap work comes up (market research, building slide decks, arranging meetings, filtering email, buying lunch) in return for exposure
2) They see you as a strong potential future hire and are thus willing to invest in training you up for 1-2 months as a sort of extended job interview.
3) They have a non-critical, self-contained side project you can come in and completely own. Examples include wireframing/designing a new site or feature, beginning a real content marketing strategy, understanding & documenting existing code & processes, etc.
#2 is the best for you, since you'll get "real" work. #3 is good, but I've seen teams give something which is a little bit too optional and then never integrate the final result, making it a worthless portfolio piece. #1 is the most boring and you'll probably have to switch companies after the stint to be considered a "real" team member, but it's probably the easiest to get into and would be a chance to trial the startup experience in general.