solved tasks

You could also try new things in your own IDE. It's like an empty canvas.

I have also wondered why we were locked out of turning tasks in multiple times after we completed it. It would be a great way to learn by trying different ideas that come to us a day or two afterwards. In the interim, I would suggest trying out Codesignal.com. It is a free site I use that gives a ton of tasks you can do, with a verification system, and example input/output variables for each task. This site does not do any teaching, but has appropriate tasks for all levels. Once you do complete a task you can see every other successful solution that people have submitted, and you can still go back at a later time and try different things. It can definitely be used for further practice.