First off, I recommend purchasing some refills of the colors you might use most. They do tend to run out a little faster than I am used to.
Here are some of my favorite pens:
Pilot FriXion Ball LX Erasable Gel Pen Fine Point (.7) Blue Ink Black Barrel (A heavy pen that makes writing nice and smooth. I replaced the blue ink with a black refill)
Pilot Frixion Sticks: These little pens are great! They are affordable and in my humble opinion write better than the pen that comes included with your Rocketbook. While they do not have a refill they last a decent amount of time, are skinnier and are available in a variety of colors.
Highlighters (Heads up you do not want to highlight text. It gets messy. Use these more for underline, arrows, etc.)
Rocketbook sells Crayola Dry Erase Markers on their site. I find they leave color residue on the Everlast. Do not buy them!