Legal (or semi-legal) tree climbing near Boston

This seems like the kind of thing my friends list might know about: Can anyone point me to areas near Boston (30 minute drive) where tree climbing is either legal, or basically accepted by the local powers that be? I've had several recommendations (Mt. Auburn Cemetary, Arnold Arboretum) but my understanding is that the custodians of those areas really don't want you doing it. Any thoughts/suggestions?