What is a substitute for maple extract?
Well, you are not going to get a maple flavour with anything other than maple extract (or maple sugar or syrup or some similar maple product).

That said, I've substituted rum for maple in some recipes and tend to think of it as a better approximation than vanilla.  The caveat of course is the amount; a tablespoon is fine to substitute, an entire cup is going to impart a pretty strong alcohol flavour that you probably don't want.

If you don't have rum or don't think it would work for your recipe, then vanilla extract is probably the next-closest thing.  However, it will change the flavour; whether or not that change is "too much" is really a matter of opinion.