This is probably way less ambitious than most of the projects here but I’m new to 3D modeling and still learning the basics. I love writing by hand and I have many pens and notebooks lying around on my desk. I wanted to solve this by designing my own pen cup and 3D printing it in vase mode (one continuous line).
My idea was to use a rounded square as a base but make a barrel from it instead of just a rounded cuboid and add little bumps, smaller ones at the top and bottom and bigger ones at the middle. My only goal was to perfectly align these bumps on the barrel-ish surface like having waves horizontally and vertically. The vase mode printing has its own restrictions too because you cannot use supports. So I really missed the parametric modeling because I needed to redo some parts several times. But I learned a lot about sweeping, lofting, projecting and making new construction planes in different ways. It was quite easy on the sides but I couldn’t wrap my head around bending the bumps around the curved corners in the beginning. I have a feeling that it could have been done in an easier way. 
All the sketches I needed:
The final “positive” elements (one column for the straight ones and one for the curved ones), the “negative” ones are behind them:
The final solid shape:
And if you’re familiar with vase mode printing you know that the thin walls are very flexible so I used my pattern to add a top piece that basically just clicks into place:
I liked this so much that I added some separators what join with connectors so everything can be printed without supports:
I even extended this to have alternate models:
After exporting all these from Shapr3D and slicing in PrusaSlicer I printed them with some matte rainbow filament, and here is the result:
No more pens lying around on my desk! 