I mean, you could add an additional volume from which case you have 1 of 2 options: move the vhosts to the new volume or expend the current root partition (or whatever partition is setup) to the additional volume. I would not recommend the second option though as there's a lot of risks involved in that. I would instead, if you go the route of adding and additional volume, is to just move the vhosts to the additional volume. You can refer to
Moving the Virtual Hosts Directory for host to move the vhosts.
But otherwise, your best option if you want to keep everything is to resize the droplet (unless, of course, digital ocean gives you the ability to simply resize the drive itself but last time I checked with digital ocean, that isn't an option since it's not like aws or azure where everything is completely separate resource and such gets charge accordingly).