Matt Mortem:
Try going to the question mark or undiscovered locations on your maps. You might find abandoned sites. Those give you automatic XP points. You will also find Places of Power, which give you upgrade skill points and augment your sign abilities.
Also, if you find monster nests, destroy them! That also earns XP points. Also, unlocking new towns and markers will open up unmarked side quests, let you get new Witcher contracts at the bulletin boards, etc. If you have mind control level too, I would recommend using that on dialogue options. Use that on enemies in dialogue options and you can avoid certain fights. It will also get you some automatic XP points.
I also recommend pursuing the Witcher school armor diagrams. Upgrade your sets as you level up too. You can find a set that will suit whatever play style you prefer. The Witcher school armors also look way more badass and augment your abilities.
One other thing. If a mission is higher than the recommended, you can still pull it off. If it's a couple levels higher, I would still say go for it. As long as you have a good strategy and know what you are doing, they aren't too insanely hard.