Intentionally selecting simple projects is the very best way to do this course. If you finish one, you can choose another.
Getting less done when you have more time is a very common issue. One trick is to give yourself less time. If you have all week to get something done, you can fill your schedule with things you want to do, leaving you with just the right amount of time if you act efficiently. Another is to start early making little preparatory steps toward the task. If you do that, it helps to schedule the second little step shortly after. The key is in making those steps small. (You can always continue with your Task if you get in a flow with it, but you don’t have to.)
I know that I sometimes like to leave household tasks half finished. When I start cleaning the kitchen, for example, I tell myself that I can leave cleaning the stove (my least favorite part) till later. But as I clean, I imagine my mother or a picky guest floating over my shoulder telling me that I can also clean up that little bit and this other one, and finally the stove. The kitchen ends up such that I wouldn’t be embarrassed if my mother popped in. And as you say, it didn’t take more than 5 extra minutes. The surprising part is that when I walk through the kitchen and see it all clean I get a burst of joy.
On the other hand, if perfectionism gets in your way, (I have the opposite problem) then you just need a different character hanging over your shoulder (maybe me!) telling you that what you’ve done is good enough. Then your reward would be extra time to do something you enjoy. It’s a good idea to have a list of these enjoyable things at the ready. Mine is an engrossing novel.