Onomappu is a YouTube channel hosted by Hitoki, a native Japanese speaker who is also fluent in English and Chinese. The videos generally have subtitles available in several different languages. Hitoki speaks slowly and clearly about Japanese grammar points, vocabulary, advice for speaking naturally, and other topics related to language learning. The way he employs learner-friendly language while still keeping his Japanese pretty natural is impressive.
Watching Hitoki's grammar videos feels like listening to your friend kindly explain something that you didn't quite "get" during class. His teaching style is engaging and personable enough to keep you hooked, which might be helpful if you have trouble concentrating on more structured content.
In particular, his onomatopoeia series is a real gem. Hitoki makes use of props, gestures, images, key words, and copious examples to really give a sense of what each onomatopoeia feels like from a native speaker's perspective.
The content of at least one of the videos may be inappropriate for kids, and the host's sense of humor might not be for everyone. However, if you enjoy more casual, conversational learning, this channel can help you get a better understanding of some nuances of Japanese grammar and vocabulary outside of a textbook.