Printed model of a parametric sine surface defined by x = sin(u), y=sin(v), z=sin(u+v) 
Interesting both to look at and satisfying to fidget with, it's approximately 6.5cm per side.

And for the technically curious

I generated the mesh using numpy and Open3d in a jupyter notebook to create a list of vertex position. To create the surface, using surface reconstruction methods caused unclean/messy results. Nested loops to grab the index of parallel vertex ‘rows’ and generating an indexed face list turned out to be much simpler and gave ideal results. Here a screenshot from the Open3D visualizer.
 

I then exported the mesh from Open3d and adjusted it in Blender to make it a printable shape without intersecting surfaces.

General 3d Printing Tips

• Make sure your printing plate is clean! Oils from your skin will cause adhesion issues for your prints. Regularly wash your print plate with a detergent like Dawn and avoid touching the surface when removing prints from the bed.
• If it's an option on your printer, always opt to level the bed before each print. If you have to perform this manually, make sure you do this after every few prints. It takes a bit of time and patience, but it's a fair trade for reliable prints.
• Ensure bed and nozzle temperatures are within the expected range for the material you are printing. Temperatures that are too high or too low will affect the print negatively by causing warping, malformed layers or adhesion issues.
• Reduce your printing speed. Slower print speed is especially important for overhangs because it gives the material more time to cool as it's deposited.