Well, assuming that glass bottles, fluorescent light tubes, aerosol cans, and song birds are not options you'll endorse, then consider the following:
1) Whatever you pick make sure it absorbs the BBs. That is, it can't be metal or stone or masonry or hard wood because BBs will bounce back like rubber balls and they can and will blind a kid. This is not just some kind of parental cliche, it's a real danger. Always insist on eye protection when using BBs.
2) Recalling my youth and the tens of thousands of BBs I shot, aluminum soda cans were always fun because they were satisfyingly reactive with that little "tink" sound. BBs would penetrate them, and if hit right they would be knocked over. Also, if you start with el cheapo brands of soda, you can give the kids the full cans, shake them up, and then shoot them to get a nice burst can.