Here are my top five writing activities that don't necessarily increase the word count, but which still help with 'writing the book':
1. Character planning
I spend a lot of time on my characters, before I even start on sorting out the plot in any major way. Obviously, the characters change over time, usually as a result of the things I throw at them, but I have a lot of notes on each character before plot-storming. I have long dossiers where I think about the influences on each character. Much of this information won't make it into the book - at least not specifically - but knowing what some of my character's first memories are, or how they would react to specific events etc., helps me get a real feel for them. If writing is going slowly, I might re-read these notes or add to them. I might think about how a character might react to something (and whether that helps or hinders them). I also find it helpful to write a quick summary of how others see the character, along with how the character sees themself.
2. Setting planning
I sometimes think I enjoy this aspect almost as much as actually writing the book! Does the location/setting match the book? Or is it deliberately surprising? For the next book (after I've published the trilogy) I wanted a lovely country house that turns out to hold a big secret. I spent a great afternoon on online estate agents' sites looking for just the right kind of house. When I posted a picture of it on Twitter, a friend said "But that doesn't look like a haunted house". Exactly. I wanted a beautiful house that turned out not to be lovely at all.
I also spend time looking at magazines like Ideal Home for the layout of a room, or the way it's styled. Sometimes it can be just a lamp that I like, or a style of sofa. But 'buying' and 'furnishing' the setting of a book can be great fun. And of course, all of those notes help with description when I get back to writing. It certainly always help me to 'see' a place before I write about it.
The words might not be flowing because I've painted myself into a corner, or I can't quite get a part of the book to work. Spending a bit of time throwing ideas around usually gets me out of it. I also find it useful to think 'where is everyone's head at?' which helps with character arcs. I find mind maps helpful. I know many people use software for this, but I use a pencil and paper (no, no one's surprised by that confession...). One of the reasons I've never fancied doing NaNoWriMo (or anything similar) is because, huge planner as I am, I couldn't write 50,000 words without stopping for planning time. I probably do some plot-storming/checking every 15-20k words, despite the fact I've usually got plot outlines written before I start writing, and know what my key scenes are going to be. I couldn't do 50k without checking I was still going in the right direction!
(or, if you can't draw, doodling).
I'm not a great artist, but doodling or sketching can open up a creative part of the brain, letting your mind work on things, even if you're not conscious that it is. According to the Harvard Health Publishing site ("The 'thinking'benefits of doodling"), if you're struggling to concentrate, doodling 'will likely activate your brain’s “unfocus” circuits, give your “focus” circuits a break, and allow you to more creatively and tirelessly solve a problem at hand.'
If it's really not flowing, take a break. Read a book you've always enjoyed. Maybe think why you like the book so much. That might free up your creative juices and the next time you sit down to write, it's easier. It works for me!