So, apparently (depending on how much you believe the wiki) we were all set to go beyond the moon after Apollo. We had plans to do a manned Venus flyby. Of course, we couldn’t have dreamt about attempting a landing, because the composition of the atmosphere and the extreme pressure of the atmosphere would have precluded such an attempt.
But, nooooo. Instead, we went the “cheaper” route with the space shuttle. In the end, of course, this turned out to be the more expensive route and the shuttle costs more per-flight than all of Apollo ever did because of the strenuous safety inspections that are required for the vehicle, and also the terrible cost of unionized contractors.
I really do hope that the ISS is a stepping-stone to exploration of interplanetary space, and that it’s not just some political tool to make nice with other countries (namely Russia).
In any case, the plans were to explore another planet, using the technology of Apollo. Perhaps this time the technology of the Constellation Program will send us beyond the moon.