I'm finally upgrading to a 1 TB SSD and i'm not sure what to do with the ol' HDD. some say they convert theirs into an external drive which sounds easy enough, but are there other potential projects?
Keep it in the PC as a secondary drive, you can move some gsmes to it which don't really benefit from the SSD, or just move them so you don't have to download them again.
It's also great for media storage, playing movies from SSD won't be any faster, and if you keep your PC powered on, you can setup a plex server or a network share, and access the movies from the TV for example. You can also make backups of your data from the SSD to the HDD, it's not as good as a backup in a separate computer, but it can still protect your data in case your boot drive goes corrupt (because of Windows updates), or simply if you delete a file by accident.