Whenever I am stuck for ideas, I usually find something to do on either one of these online resources. They are great practice tools and usually help when I am struggling to make decisions of what to do or what colours to use of what I want to practice this time around, which is almost always.
https://line-of-action.com/
Line of action describes itself as a practice tool for artists and is an AMAZING website.
It has an intuitive layout (so is easily accessible), has a well structured forum in which a lot of skill related questions have been posed (and answered), as well as great practice function which lets you choose to do gesture drawings from photographs. You get 4 different categories (3 human related, 1 animal) and you can decide if you want the pictures to be shown with a set time interval or lecture style. You can also choose specifics to the models, which is great.
http://www.posemaniacs.com/
Posemaniacs is one of the more known pose tools out there.
A bit creepy at first, as the poses don’t actually have skin on them, you can actually see how some of the muscles work in order to make that pose work the way it does. However, this tool was made digitally, specifically for mangaka and is therefore not anatomically accurate. A COOL feature is, once you open a window with the pose you are interested in drawing, you can actually use your mouse to rotate it from side to side and see other angles yet again.
This site too comes with gesture drawing features, such as 30sec drawings, negative space and random pose viewer to mix things up a bit.
https://www.magatsu.net/generators/art/index.php
A ‘simple’, easy to use generator for drawing or writing. Depending on the how creative you feel that day, you can choose 1-10 prompts which are either ‘simple’ or ‘complex’. And if you are not happy with the results, just generate new ones.
http://blog.sockenzombie.de/art-ideas-generator/
Another idea generator, although specifically geared towards artists. This time, you can choose your own categories in which you want to get prompted. It allows 0-5 promts per category and has a TON of different options in each. If you are prolific in German, you will find the rest of the website useful too, which has a lot of tips from general drawing advice, to how to fill a sketchbook, various challenges and the like :)
So yeah, these pages are my little treasure troves, if I am really stuck for something to draw. Worst case scenario, go back to basics and practice basic skills like shading, placement and perspective. Any time you pick up your tools is practising your craft. So get out there and get creative.