At some point in David’s reign, there was a bad famine. When David had the priests inquire of God, they found out that the famine was punishment for Saul’s attempt to genocide the Gibeonites.
So David called the Gibeonites and was all, how much silver and gold will it take to erase the bloodguilt? But the Gibeonites were all, We don’t want your money. Only blood cleanses blood. We want to sacrifice seven of Saul’s male descendants. David was all, Fair enough.
Now David couldn’t bring himself to sacrifice Mephibosheth or his kids because of David’s love for Jonathan, so he tracked down some of Saul’s daughters and took their kids–three from Rizpah and four of Merab. (Merab, by the way, was married to Barzillai’s son, so the whole exchange between David and Barzillai during Absalom’s exile was of political importance).
Then the Gibeonites executed Saul’s seven grandsons on the first day of barley harvest, and lo, the famine ended.
Now Rizpah mourned so piteously for her sons and nephews, staying out in the fields to guard their bodies, that David felt really bad. He took their bones and Saul’s bones and had them all interred in the tomb of Kish (Saul’s father).
Also David had stopped going out to war because once while the Hebrews were all out to war with the Philistines, David became exhausted and almost died during battle with Ishbi-benob (one of Goliath’s giant relatives), but Abishai saved him. So all David’s men decided that he was too precious to risk in battle anymore.
There were two other Philistine giants killed by the Hebrews, (besides Ishbi-benob and Goliah, who may have been killed by a guy named Elhanan): Saph, killed by Sibbacai and this other dude with 12 fingers and 12 toes killed by David’s nephew Jonathan.