From there, I would suggest Kanji, especially those used in words you know well - it‘s very likely you know those kanji already. Not the best next step if you want to do a jlpt test anytime soon, but surely a fun way.
I would always encourage everybody to choose fun over jlpt.
You can filter the kanji dictionary word examples to words „you know“, so you can easily spot kanji that you have come across quite often already. Those are wonderful quick wins. 🙂
