Here's what I've found useful to store on my SD card:
- Pictures/video
- Google Play music and movies
- Youtube and Netflix downloads
Some apps can be moved to the SD card, but most can't. You can go to the app settings for individual apps to find out if they can be moved, or you can just download an app from the Play store which will give you a list of apps that can be moved, and do the move in bulk. Apps run a bit slower from the SD card, but back when I used to put apps there I didn't really see a noticeable difference.
Note that if you put photos and video on your SD card, someone could potentially remove the card from your phone and access them, even if your phone has been locked. If you're worried about the safety of your personal data, you can encrypt the SD card easily from your phone's lock screen security settings menu. You'll still be able to access your SD card data from a computer when your phone is plugged in and unlocked, but if someone ever steals your phone they won't be able to remove the SD card and access the data. If you ever switch phones you can decrypt the SD card just as easily. However, when I did this while switching from my S7 Edge to my S8+, it failed for some reason and I lost all my photos. I also had to exchange my S8+ under warranty recently, and precisely the same thing happened when I decrypted my SD card. So if you opt to do this, you might want to back your data up to a computer hard drive before switching phones.