For a long time, I would get in these super ambitious moods, where I would have 500 things I wanted to do and didn't have the time to do all them. But then a few days later, I would end up sitting on my ass bored with nothing to do, and those ambitions wouldn't even cross my mind. I would sit around and accomplish nothing for hours, all while feeling bored.
I started writing everything down that I wanted to accomplish whenever I got into the ambitious moods. Whether it was short term goals (like cleaning, yardwork that I wanted to do, a craft project or the like, little things I always meant to fix around the house but never seem to do), or more long-term goals (exercise 3x times a week, study a language a few times a week). Then, whenever I got bored and ended up sitting around watching TV or browsing the internet... I looked at the list of all the things that I really wanted to accomplish. And I picked one.
I started getting so much more done. I put off less things.
I ended up with a huge whiteboard that looks like one of those word clouds (some words big, some small, in no discernible pattern) where the bigger things were more important to me. I erase things as I complete them, and I add new things constantly. Some items still get procrastinated on and stay on the list for a long time... But things are constantly getting done. It's fulfilling to erase something from the board. The reminders help.
Downtime is still good. Sometimes I just want to chill out and veg, and that's ok. But sometimes I would sit around and be discontent with just sitting around... but wouldn't be able to think of anything to do. It helps with that.