http://www.amazon.com/Mountainsmith-Ghost-Backpack-2800cu-in/dp/B00076Y2V0
I used one of these last summer for three weeks in Guat and Honduras. My 19 year old daughter and I went. She had a small North Face pack, it was an old one of mine, about the same size. Her pack worked better but mine was ok. We both carried on, no baggage handlers for me.
I have found that you need no more for three weeks than you do for one. Getting things washed is cheep and gives a good reason to hang around town. We left wash and went to Tikal over night. That would be a must do for me. Stay in a hammock over night at Tikal and see the sunrise in the morning.
Remember less is enough.
The old adage about travel still works.
Take all the stuff you want to take and put it on the bed. Put the money you are going to take there as well.


Now pack half the stuff and twice the money.

Have fun.
K