A fun way to give yourself a bit of a challenge with a Zentangle tile is to use only a single tangle pattern. I find this forces me to be more aware of scale which I tend not to think of enough. You can do some very small tangles and some much larger ones.
Here's a tile using only Coaster (thanks Carole Ohl!). Well, except for a tiny spiral at the top. You can see how the size of the tangle makes a difference.
This one uses only stripes in
various directions and widths.
Here I used only the tangle Drupe (thanks Rick and Maria). Instead of varying the size, I tried a different fill in each section.
And finally, here's quite a simple one using only a drawing technique called stippling - all tiny dots done with the tip of the pen.
You can see one of Shelly Beauch's tiles using only Umble here.
I had been going to suggest that you choose a favorite tangle, or one you've never done before, and do a tile entirely with that pattern. But Laura beat me to it! This is the zentangle weekly challenge number 11, and the technique now has a cool name: Monotangle. Have at it!