Originally Posted by
Scifience
Pretty much agree with what everyone else has said. The key points are:
1. Don't mention volunteering on a farm. You're a tourist who happens to be staying on a farm. Bring proof of sufficient funds as though you aren't getting free accommodation.
2. Don't screw up and tell immigration you intend to stay for longer than 90 days.
3. A couple days in Korea/HK/etc. around the 2 month mark won't, in my experience, be suspicious at all. Come back, you're still a tourist, get another 90 days. Unless you've done this a bunch of times or are otherwise suspicious, you're extremely unlikely to get questioned. Pushing your first entry all the way to 90 days is likely to get more scrutiny than "re-upping" somewhere in the middle.
If you stick to these points, it's exceedingly unlikely you'd have trouble.
+1
Also pay attention to clothing and appearance. Have an onward air ticket. Carry cash or travelers checks and a credit card. Write down a relatively short length of stay (e.g. 2 weeks). Put down the name of a decent hotel for your address (might want to have a booking you can cancel.)
That said, even if you don't follow these recommendations, 99% chance you will be fine. The authorities are only worried about people working or engaging in other illegal activities.