Just a small heart shaped box. 35x33x14mm
Made on Onshape. Feel free to edit it :https://cad.onshape.com/documents/a523e35430889b5cf83175eb/w/a068dbca4621fbfd3084d775/e/4e2041a20337838b90108521

Update:  Based on suggestions from other makers I have added a 3rd version with trim tabs on the base and above the gear mechanism to stabilize the pedals while printing.  If you don't mind a little trimming after printing give the "Trim Tab" version a try.  Hopefully this will be a much easier print.
Update:  Added a 2nd version with a thicker tapered support for the heart.  Some people have had issues with the heart stem failing during the print.  If you run into this issue or don't mind a heaver support please give the "Thick Heart Support" version a try.
Re-mix of my original blooming flower toy with a heart center for valentines day.  
https://youtu.be/GoKa3Kegb-w
Original design:https://www.thingiverse.com/thing:3400477