Well, even Genki describes itself as an integrated course in
elementary Japanese, so it's only logical that there's still much to do after completing it.
For learning a base list of kanji, I personally used the KanjiDamage website - I can recommend it. It's somewhat crude, but
very pragmatic and not afraid of pointing out characters that look extremely similar, have strange irregular pronunciations in certain words, only have limited use in practice and so on; all helpful things which more formal learning resources like Genki don't do. Oh, and unlike WaniKani it's free and doesn't even require registration. That said, even after completing this list you'll still keep encountering new kanji frequently for a long time to come.
Concerning pronunciation and quick sentence building, you could find a speaking practice partner on a site like GoVoluble (was advertised by its creator here on JREF some time ago). Apart from that you can switch your internal monologue to Japanese: when you mentally react to something you see, when you're pondering on something... do so in Japanese thoughts instead of in your native language.
I could stop here, but it surprises me that you didn't mention anything about grammar and reading/listening practice; with just Genki under your belt you won't get very far. I can recommend "An integrated approach to intermediate Japanese" as a followup textbook, it introduces a lot of useful extra grammar not covered by Genki and has only minimal English text.
For listening practice there are plenty of options. Personally I was watching English "Let's Play" videos before I started learning Japanese, so I started watching Japanese ones as well. If this is something that interests you, you just have to search for 実況プレイ on Youtube or ニコニコ動画. (Do
not search for レズプレイ like I did at first, that's something completely different). Later on you can switch to something more difficult like
the news.