Note for anyone else looking for how to use checkboxes in markdown mode: You can have a list of plain clickable checkboxes if you put a * before each one, making a bulleted list. You also have to have a line of plain text before the checkbox, and do it all in one block. Pretty inconvenient, but it works.
Like:
subtasks
* [ ] do a thing
* [ ] do another thing
* [ ] do the last thing
Looks like it also works with + instead of * as mentioned above.