The most obvious example would be the series of summits with Mikhail Gorbachev beginning in 1985. These talks, aimed at slowing the arms race through strategic arms agreements, helped foster dialogue with the Soviets after the collapse of detente during the early 1980s. Despite many setbacks and disagreements, dialogue and negotiations with a leader of a nation Reagan pronounced an "evil empire" in 1983 were a significant step forward.
A major issue during Reagan's time was the holding of hostages by terrorist groups in Lebanon. Reagan had to decide what to do about those hostages. He eventually decided to sell weapons to Iran, hoping that Iran would then help to persuade the terrorists to release the hostages. That was a type of diplomacy.
I have to admit the one thing I remember from President Reagan's time in office was his demand for Gorbachev to tear down the Berlin Wall. While I was under ten when he entered the office, I decided at the moment he took office that he was going to do something great.
"Mr. Gorbachev, tear down this wall!" Reagan's speech at the Brandenburg Gate of the Berlin Wall served as the bookend to Kennedy's speech at the same place almost 30 years earlier. These two events highlight the height and the conclusion of the Cold War.
One of the most important diplomatic issues during Reagan's term in office involved negotiations about his so-called "Star Wars" missile proposal. Reagan hoped that defensive missiles designed to shoot down offensive missiles would allow for a huge reduction in the number of offensive missiles possessed both by the U. S. and by the Soviet Union. Soviet leaders were suspicious of this proposal, and, at the summit in Iceland, negotiations broke down and both leaders went home.