Not had a chance to give this a whirl myself yet due to a nasty dose of manflu and work-excess, but it sounds as though it could offer nuggets of wonder. Platinum Arts Sandbox is a free world-builder tool, apparently so easy and quick that even those screaming, sulking, pre-pubescent monsters who are supposed to be the future of humanity can use it. And make scenes like this:
Aw, pretty. Kinda. The worlds that can spill forth from your boundlessly creative hands don't, as I understand it, offer much beyond roamability, but there's a little of Garry's Mod in its claimed drop-in approach to adding models. Garry's Mod but for scenery and without all the juvenile teabagging poses? Could be quite promising, really.