A sling. Or three. They're like a silver piece, buy a dozen of them. They're small and concealable, and between those and your natural weapons, you'll never lack a weapon. Use one to tie your hair back and they probably won't even take it away from you if you're captured.
Playing a monk is really fun until you run into something that flies or can rip you up if it gets a full attack on you, because monks are all about full attacking. Something like that takes you down to 1 or no attacks a round. What you need then is a missile weapon. Crossbows also work, but don't seem very monklike to me. I like the sling, and oil of Magic Stone isn't too expensive.
Kamas can be used for tripping, nunchuku and sais for disarming. A staff can be used two-handed for 1.5* your strength bonus; at low levels, when your fist does only 1d6, if something is hard to hit, you might as well use the staff. You can throw clubs if you have a couple of them, and they're free.
In any case, always have a weapon in case there's something you don't want to touch with your hand, like a black ooze.
If you're going after spring attack, get it as early as possible, like with your level 6 feat, because once you get higher level you'll have better things you feel like taking. If you're not, don't bother taking mobility, it will almost never be useful since you'll be maxing out tumble.
With your bonus 6th level monk feat, pick up Improved Trip, not Improved Disarm, because your fists are light weapons and take a penalty compared to most weapons opponents will have anyway. Unless you really like the sai, which has a +4 to disarm and is light, so it evens out with a medium weapon, and then your bonus from the feat would put you higher.
Flurry early and often. From first to 4th level, the only time flurry results in lower average damage for a round of full attacking is when you have to roll exactly a 19 to-hit. At all other times, it is as good or better than your single attack.
Don't forget to use your stunning fist feat; if you're fighting a wizard or something with bad fort saves, just declare it before you roll to attack, and it activates automatically if you hit.