Draw a skyline - it can be a city, country, the horizon along the water - any skyline you can see in your mind. Create a doodle and put that skyline on your paper or computer. Don't spend a lot of time - there's no need to follow the buildings of an actual city or mountain range. Let the skyline flow from your mind, through your arm, to the page.
I used to tell my students to create a line along the tops of the buildings - a jagged line zigging and sagging along the page. Then they dropped the verticals and turned each one into a separate buildings filled with Zentangles or windows or stripes. Your skyline does NOT have to be in perspective or have any depth to it. Try not to overthink it. Another way to approach it is to draw one building (or rectangle) on the page then draw another rectangle next it or slightly behind it - keep layering behind each rectangle. Add windows if you have time. Each rectangle can be a solid color or keep them all black. If you only have a minute or two - create the outlines of the buildings and leave them empty. These are meant to be quick, carefree.
Don't forget to join the Facebook Group (if you haven't already). There, you can post your doodles, check out other doodlers creations, give and get drawing tips, learn about software, post and see links to videos and websites for doodlers, and much more...
Here are some skylines for reference: