Apollo program - What could have been

The Apollo program originally called for at least 3 additional lunar landings after Apollo 17.   Over time, Apollo 18, 19, and 20 were canceled.   In addition, a follow-on order for additional Saturn V boosters was canceled by President Johnson in 1968 before the Apollo 11 landings even happened.  
If the missions had not been canceled, here's what additonal landings would have looked like.   

The "LM Truck" and the "LM Taxi"
Extended lunar exploration was desired, but the limited consumables that could be carried by the Lunar Module (LM) limited the stay to 3 days at a maximum.   In addition, since the LM could only carry 2 people a 3rd astronaut was required to stay in orbit and the consumables on the Apollo Command and Service Module (CSM) would be limited for them as well.
The idea was proposed for a manned or unmanned flight to deliver a LM containing supplies for a 7-14 day stay.   This vehicle was designated an "LM truck".     Later, another mission would deliver 3 astronauts to the surface using an LM equipped to handle 3 astronauts.   This was known as the LM Taxi.   Precision landing near an existing previously landed spacecraft.   This capability was demonstrated on Apollo 12 when Pete Conrad landed their LM near the Surveyor 3 probe, which was landed on the moon 2 years earlier.
Further follow-on equipment that was proposed would have included a 6 man lunar shelter that could be expanded with each flight.   
A permanently occupied lunar base with rotating personnel could have been established well before 1980 which would have no doubt led to an eventual international co-operation on the moon.

Saturn V enhancements
A 2nd generation of Saturn V boosters would undoubtedly had improvements in engine technology.   Had it been followed through to the current day, we would have seen lighter and stronger materials and enhanced electronic systems allowing for larger payloads.  
But what about the Space Shuttle?    It would have still existed and been used.   The original plan had been for the 2 to co-exist.   The Shuttle would provide human transportation to low Earth orbit and the Saturn V would have provided heavy lifting capability.    Had we not abandoned Apollo, we could have saved Skylab, expanded it, and had the equivalent of the International Space Station decades earlier at a much lower cost.