If you choose to multiclass, you'll likely want monk 5 first, for extra attack.

Sun soul + druid can be fun if your DM agrees that you can use the sun soul ranged attacks in beast form (RAW, it should work just fine). You also get unarmored defense/movement, ki abilities, and deflect missiles in beast form, as well as slow fall if you get turned back into humanoid form while climbing/flying.

That's the only monk multiclass I really like, aside from maybe some level 1 dips. You can pick up 3 or 4 levels in your favorite fighter/ranger/rogue subclass and it'll be totally fine, but... I guess I don't feel like you're gaining all that much from being a monk at that point.