How do you get decent experience when all quests/contracts are grayed out?

I just finished all main quests in The Witcher 3 including the DLCs, but I'm still four levels away from reaching level 100.

All secondary quests, contracts, and treasure hunts are grayed out due to my higher skill level, so completing them would net me very low experience points.

Do things like question marks on the map give more experience than grayed out objectives?

You can't. The game is designed to scale the EXP based on your level. You can turn on enemy level scaling, which are capped at 105, so killing enemies would be your best bet.

If you're playing with mods, there is a mod for just this problem.

Witcher 3 Always Full EXP Mod