I like to change money at FX Traders which is inside the air-conditioned HI-LO grocery store at the Sunshine Plaza.
You'll need I.D. and you'll get a good rate.
Do not change money with the touts outside the store who approach you.
Yes you can change the money back to US when you leave, if you have surplus.
Change a couple of hundred to begin with.
You'll probably spend that on taxi's, drinks etc in 4 days.

You won't have to 'flag' a taxi outside Treehouse. You'll be honked at; gestured to; asked if you want a taxi. Negotiate before hoping in.
From Treehouse to lets say the HI-LO plaza; I wouldn't pay more than 500J for a charter ride. Thats about 6 dollars; far cheaper than a cab in your hometown for the same distance. That's on the high side.