We are all very different and therefore we have problems with very different aspects of learning German. An online quiz or a text book doesn’t know your weaknesses. For that reason I will recommend making your own quizzes once in a while focusing on the exact problems you have. It can be grammar, vocabulary, it can be everything…
The one I use is called CueCard, but there are many other alternatives:
Record German tongue twisters and see if you can repeat correctly.
Make learning grammar less boring and more effective… There are so many ways to use this tool. Be creative and you can practice almost everything you can imagine.
And remember to save your quizzes in a way so they are easily found and quickly accessed, for instance on your desktop. I often play solitaire or hearts, when I have nothing to do, but just as often I open one of my self-made German quizzes.