This is a great parsha for many different reasons. This is the parsha with the three angels, when Sarah was promised she would have Isaac, the destruction of Sodom and Gomorrah, Lot and his salty wife, Lot and his daughters, the travels of Avraham and Sarah, the birth of Isaac, the sending away of Hagar and Ishmael and the binding of Isaac.
There is so much here to choose from.
I want to start with a side note: We start this chapter with G-d visiting Avraham. Everything is in the Torah for a reason. This is a one liner. "Hashem appeared to Avraham" - why was G-d appearing (not literally, G-d doesn't have a physical form) to Avraham? Because He was visiting the sick. This is important since if G-d takes the time out of His schedule to visit the sick, there's no doubt that we too must take time to visit the sick as well.
In Chapter 18:2-7 Three angels were sent to visit Avraham after his circumcision. The moment Avraham saw what he thought were men, he ran to greet them. This is a fellow who only three days earlier had a circumcision! Then Avraham hastened to Sarah and asked her to make cakes. Then he ran to prepare a calf to eat. All of these are hurried actions to feed the guests. Run, run, run.
What we can learn from this is that we should not let a mitzvah get cold. In the words of the Sages, "When a mitzvah comes into your hand, do not let it pass."
There are so many opportunities that pass our way that we don't take advantage of. We figure that we'll catch the chance when it comes by our way again. Whether it's keeping kosher or Shabbat, or being friendly to a brother or sister, greeting another with a smiling face, going to a Torah class, visiting someone who isn't feeling well, feeding guests, or just cleaning after ourselves so someone else doesn't have to, we cannot let these opportunities pass us by.
Rabbi Hillel said, "Do not say, 'When I have free time, I will study' - since you may never have free time." (Pirkei Avos/Ethics of the Fathers 2:4)
This is a very true statement. Free time always seems to get swallowed up by all sorts of mundane activities. We need to take the time to grab that mitzvah walking by.
[Some information is taken from Don't Look Down by Rabbi Michael Haber]