I don't know much about other English-speaking countries, but there are ample opportunities to volunteer in the US. Boys & Girls Clubs are a good start - you just have to watch out for the expected paranoia. Generally kids will do their homework and then go play when they're finished, you can help with homework and then play and have fun.

Tutoring - I love tutoring and this is so much easier than teaching. Teaching is great too but only if you have a good class that's reasonably well-behaved. I've found this can vary enormously by country. Tutoring tends to be very relaxed and low-key.