I'll give 3 of my thoughts, that don't break the bank, and I have found useful over the years:
1.) Cargo Organizer: I have a lot of stuff in this thing; jump box, cables, tow strap, extra d-rings. first aid kit, code reader, umbrella, rain jacket, blanket and more.....It ties down really solid.
2.) Inserts For Door Pockets, Cargo Pockets, Cupholders and More: These inserts really just finish things off, but I've found them useful.
3.) The center console organizer someone else posted below, so you can follow that link.