Spare foam earplugs and a few mints. I discovered, after putting my insurance card in there, that it's not watertight...
I don't have this compartment on my GS but on all my bikes I either laminate my insurance and registration with a cheap home laminator or put these paper documents in a zip loc bag and stow them on the bike. Either works fine.
'16 R1200GS Triple Black
'16 KTM Super Duke 1290R
'18 Yamaha WR250R