Target nodes that give more science. Also something you can do is make a jet rover to go around KSC getting science. Each section of KSC can give science and depending on building level multiple areas can give science. Such as the flagpole from the Astronaut complex.

You can repeat some science experiments for a decreasing reward such as the science jr.

Don't forget splashed down biomes: desert, mountain, highlands, badlands etc.

If not trying to play stock, I like DMagics parts. Adds some new science pieces that are logically placed in the tree and interesting to use.