If you're not already familiar with ES6, I'd recommend picking it up. Maybe do a project in React Native? :D That really teaches you some useful skills.
For C/C++, what sort of topic are you interested in? If you want to get a bit low-level in C/C++, try this: https://scs.hosted.panopto.com/Panopto/Pages/Sessions/List.aspx#folderID=%22b96d90ae-9871-4fae-91e2-b1627b43e25e%22
It's pretty challenging and definitely delves into the more complicated side of things. I'd strongly recommend getting or finding the associated CS:APP book since it contains the challenges that make the course what it is.
Can't really help with Java since I don't know of any super-challenging OOP review things out there.