You have to be careful with strong concentrated acids and alkali. Something like a car battery exploding close to your face could easily disfigure you for life, the acid in there is pretty concentrated and is especially dangerous for your eyes. Skin is actually somewhat resistant to it, as long as you wash it off REALLY quickly (i.e. jump into a pond or hose it off right then and there).
One trend here a few years ago was making bottles explode using a mixture of dissolved lye (NaOH) with bits of aluminium powder. This reaction is quite exothermic and generates a lot of hydrogen gas, creating enough pressure to make a capped soda bottle explode quite violently.
This is not very dangerous if done in a remote location and such, but kids were doing it on the street here, and in some cases people passing by got injured by the hot concentrated solution of lye flying around. Afaik they even stopped selling drain cleaner to minors for some time to prevent this.