What is the best way to allow one-time freelancers to access my droplet? Currently, I'm using ssh key to access my droplet. Instead of giving the public key to the freelance developers, I'd like to know which way would be the best to grant them temporary access.
It really depends on what they'll be doing.
If they simply need to upload files, such as during the development of a theme, some code, etc, then SFTP would work by providing low-level access to simply use a application to upload.
If they need to install actual software to the server, there's only two options -- root or a sudo user (with sudo being the preferred). I'll always recommend creating a sudo user and setting it up with both an SSH Key and password (as the password is needed to elevate permissions).
If it's a different case, please detail what you expect them to be doing and we can provide feedback.