Top exterior pocket: Nothing. As you can see, it's zippered shut. It's small enough that you might not need further organization. That said, you could fit a Pocket Pouch here or add an 8" Key or Utility Strap to tether your keys, flashlight, knife, or other small items.
Exterior water bottle pocket: Travel Stuff Sack, Size 2, in Iberian.
Main compartment: Two Tri-Star Medium Packing Cubes would fit, but we chose instead to use one Tri-Star Medium Packing Cube, one Tri-Star Small Packing Cube, and a Travel Stuff Sack, Size 2, in Wasabi. We liked how the Travel Stuff Sack fit nicely on the other side of the Tri-Star Small Packing Cube. Yet another option would be one Tri-Star Medium Packing Cube and two Clear Quarter Packing Cubes (one in front of the other). And don't forget the open-top pocket inside of the main compartment of the Synapse 19: put your tablet (in a case or sleeve) here, or a Medium Organizer Pouch. Last but not least, there's a 3D Clear Organizer Cube in Black, tethered with a Key Strap in Wasabi, that'll fit/rest on top of the Packing Cubes.