Keyrings make popular souvenirs, they are cheap, useful, and colorful. They also act as great reminders of where the person received them. Nonetheless, when looking at keyring suppliers make sure that they make a good product before actually buying any keyrings.
The image used should be bright and colorful, with no signs of pixelization; the image used on the keyring should look great for its size. The fob and the ring itself should be well connected; it should not be easy to disconnect the two. The ring itself should not be hard to open; if it is impossible to open then it cannot be used, negating its usefulness. Also, check on the reputation of the company itself; you want to make sure that it does deliver a good product and on time.
Those tips should allow you to create the best possible keyring for any purpose, promotional or souvenir.