I think it's fair to speculate that the second term of John F. Kennedy would have been pretty mellow and not been as productive as the Johnson presidency was.
If Kennedy survived Lee Oswalds attempt on his life in Dallas in 1963, he would have not done much in 1964 but to gear up for his reelection campaign that he would have won in a landslide over Barry Gold Water. While the Gulf of Tonkin incident still happened Kennedy's priorities would not have been to escalate the war in Vietnam, he more likely than not would have simply blown off the supposed attacks by Vietcong gun boats as bad intelligence and waited to dig deeper before starting a larger war. Besides without the paranoia and shock caused by the assassination of Kennedy and the thought that JFK was killed as part of a communist conspiracy Johnson had and never truly dismissed, chances are the war would not have escalated much.
Starting in 1965, Kennedy would have begun to petition Congress to pass a civil rights act to end racial discrimination in the South, but would have been opposed on both sides of the aisle for political reasons. Only with Lyndon Johnson's support would he have gotten it passed. The same is true for a voting rights act by 1966. Fighting this battle also meant Kennedy would have likely started gradually pulling military advisers out of South Vietnam while trying to wind down the Cold War, removing the 16,000 or so there only by the end of his second term. Kennedy may have also looked at ways to expanding detente with the Soviet Union by trying to get a joint mission to the Moon planned, that would have ultimately probably have been rejected. Instead a realistic alternative is an additional extension of the limited test ban treaty signed when Khrushchev was the premier of the Soviet Union.
These measures would have infuriated many conservatives whose vote would have turned towards Richard Nixon running in 1968. If the war in Vietnam had turned against the South, chances are the GOP would have taken advantage of the paranoia of Saigon falling to the communists and another arms treaty with the Soviet Union to attack Kennedy.
Things like medicare, medicaid, the Great Society that reduced US poverty by half, would not have happened. If a Robert Kennedy, Lyndon Johnson, or Nixon presidency followed JFK's this might have changed, but not likely. But unless Nixon ran and won in 68 and had a chance to escalate US troop presence in Vietnam, imagine the greater stability it would have brought.
Race riots, women rights, the energy crisis and oil embargo of the 1970s would have continued. But without Kennedy implementing a draft for Vietnam, war protests would have not been as severe. No recession would have followed the Vietnam War, a much less troubling energy crisis, and no Vietnam syndrome to lead to the Soviet Union taking advantage of America's weakness and invading Afghanistan in 1979. The Iranian hostage crisis was likely though.
