I have a few HDDs which contain backup data. I want to store them in a secure place. I know some good practices like storing them in a different building so if there is, let's say, a fire, you can still restore the data. But I am more concerned in the degradation of the HDDs, for example by humidity. Should I store them in a plastic bag? Is there any special way to store the HDDs so they can not be damaged by the enviroment?
I agree that a NAS is always the best answer for external storage. If you truly want "Off site", I would recommend a safe deposit box. If you are looking to store this long term and only touch it in the event of a catastrophic event, you could use a zip lock bag or even vacuum seal it before storing it in the safe deposit box. If you use any sort of on site media rated (class 125) safe, you will need to include some sort of desiccant in that safe to keep the humidity to a minimum.
Another lower budget option that is accessible would be to put a NAS in a different physical building with a network connection that is either fiber or point to point wireless.
For the case, I see it would be beneficial to consider 3-2-1 rule with RTO and RPO in order to get planned Backup and DR sites.
For a backup and DR boxes, I am using 2x Synology, so I can suggest you to take a look what they got. Veeam Backup software is used to backup/move data to backup NAS and then to DR one (that is in other building).
I found the hardware solution from aclouda that seems to be more beneficial for your caser. I see it allows to use cloud storage as local SCSI drive thus it allows to run backup jobs to cloud as it would be local drive.
Don't store your backups in off-site hard drives. That sounds like a dangerous and inefficient way to store off-site backups.
I'd store your on-site backup on a professional NAS or SAN with disks in RAID (based on whatever you can afford) and then use a cloud service for off-site storage. A cloud provider like Backblaze (no, I'm not affiliated with them in any way) is really quite a cheap way to store an off-site backup when you consider the costs of purchase hard disks, upgrade, and maintenance. You would just upload your off-site backups.
Should you stick with hard drives off-site, store them the way they should be stored, in an off-site server in a RAID configuration and upload to that from your primary site. Moving mechanical disks on a regular basis isn't a good idea.