There used to be a way in the Apple Store app to swap between cases, bands, and faces so you could see how specific cases and bands would look with different faces. I don't see that there anymore, but you can choose from many different faces. As others have said, there is no way to install third party faces, but Apple provides quite a few. You can customize things like colors, complications, and dials... so the combinations allow for quite a lot of options. There are also photo faces where you can make a face out of a photo or a series of photos.
You can read about some of the different faces on this and other websites. Just search for "Apple Watch faces" to see some examples.
If you like the face they showcased yesterday, but find it too busy, you will be able to hide some (or even all) of the complications. You might be able to change the style of the dial or colors as well.