Hi Emma,
I am an Amigos veteran who went to Uruguay last summer for five weeks. Anyone who you talk to will tell you that no two projects and two volunteer experiences are the same, so what I tell you will likely be very different from your experience this summer. The shorter five or six week projects are great if you go back to school earlier than most people or have other plans already laid out for the summer. Regardless of where you go, your time there will pass very quickly and you'll probably look back on it wishing you could stay even longer. Yes, the structure of the projects is rigid in that you must stay within your host community, unless granted prior permission. There isn't the time and resources to have that freedom to explore independently. My best advice to you would be to contact former Amigos volunteers in your area to talk to because they will be more than willing to talk to you,and it's better than me typing a long winded message. I hope this helps!
Antonia