I want to download my droplet for later use and close down my account

There isn’t a download option for snapshots. but you can do is create another larger droplet (in terms of storage) and then dd-stream via SSH to that new droplet and then download the resulting .gz file to your computer:

ssh root@new_droplet_ip "dd if=/dev/sda | gzip -1 -" | dd of=image.gz 

Then download image.gz to your computer and terminate the account/droplets.

Cheers