at least for the next decade and concentrate on Terrestrial Planet Finder and similar projects:
We've already found hundreds of extra-solar planets by indirect methods. Now imagine direct visual observation of an Earth-sized planet within a habitable distance from its star (even as a tiny disk). Imagine we get just enough spectroscopic data to confirm traces of oxygen in the atmosphere. Given the sheer number of planets being found, I'm sure this can be done within a few years, given a project like TPF.
That will do more to re-ignite the world's interest in space than anything else currently being planned, including any Moon/Mars missions. Not only is that the best way for NASA to secure its financial future, but it may also help world peace. Because it will be instantly recognized that humanity is now only one more step away from detecting other life in the Universe and only two steps away from detecting other intelligent life. Such a realization can change the way we look at each other (especially the way we look at each other's differences).