There's a video going around of the military disposing of sodium in Lake Lenore, a lake that was already highly alkaline. I saw this last week on GuardianUK from a tweet by @GrrlScientist.
Phil Plait saw that video too, then tweeted a link to this website with more alkali metals reacting with water. Don't get bored by the corrosion part. Wait until they put the cesium in water.
It reminds me of the risk of having pure elements on hand to play with, and the stubbornness of some people when it comes to taking science seriously. When I was in the 8th grade my science teacher, Mrs. Burns, had some sodium. It was a few small pieces in a bottle of oil, like in the video above. The wood shop was in the next building at a right angle to the science lab. For some unfathomable reason the shop teacher did not believe that sodium had a strong reaction to air. And for some slightly more fathomable but distinctly nefarious reason Mrs. Burns decided to convince him that it did. So she and the shop teacher went out to the lawn between their classrooms with a cinderblock, the jar of sodium, forceps and a hammer. The students stood at the windows of the science lab looking on. Mrs. Burns reached into the jar of oil with forceps and pulled out about a 1/2" chunk of sodium. She placed it on the cinderblock and quickly backed far away. The shop teacher, whose name I can't remember at all, swung the hammer back over his shoulder and hit the sodium. At which point the hammer reversed trajectory with a suddenness and force that the shop teacher was not expecting whatsoever. The sound it made hitting the brick wall behind him would have surprised him more if the concussion of the sodium explosion hadn't made him deaf.
It's good we have YouTube now so shop teachers don't have to suffer temporary hearing loss to learn a lesson. It looks like 10 grams of sodium costs about $65 now so that was an expensive lesson. 10 grams of Cesium is about $200 though, so at least economy offers a level of safety for shop teachers when they deny science. Also there's a $75 Hazmat shipping fee with any purchase of alkali metals. They must be signed for by an adult.