Paradoxically, physical activity. It's like playing chess; theoretically only mind is involved, but you must be in good shape in order to win tournaments. 

I prefer to combine grammar studies, pronunciation exercises and passive exposure through different media with talking to real people at various events. Sometimes, overwhelmed with too much studying, I find myself lacking motivation. Then, after a strong dose of sport and activity, I am able to cope with new challenges. 

I agree with what Romnik said.

Reading books, if you enjoy it is a great way to keep your level.

But if you don't enjoy reading, you can try so many other things.

-Follow native accounts on Instagram, Snapchat if you are into shorter spans of study.

-Short You Tube videos on topics that interest you are a great way to continue getting practice. Anything goes, like cooking, fashion,  sports and dancing.

- Reading shorter texts, like maybe blogs. Blogs in English are easier to read than a book and they are shorter. The language they use is conversational so it's really useful !

- Listening to music in your target language. You can pick a kind that you like and try to understand the lyrics before checking them out online. A friend of mine, learned English very well this way !

Personally, I use Instagram and Snapchat a lot but I also enjoy watching TV series on YouTube ! Hope you find something you like to keep you engaged with English !

if you want to learn any language,you must love it and have a goal to achieve .Search the fun ways to learn language like by music , fun stories ,talk with native speaker , it's independent to you. I mean what you enjoy doing .

And Good Luck  :)