Real volcanic eruptions can be dangerous. Simulating them can be hazardous, too. Using a big garbage bin full of water and a 20 oz. soda bottle with a few mL of LN2 works (very nicely), but the LN2 can be hazardous since it's so cold, and the pressurized vessel essentially turns it into something like a bomb.
This kind of eruption is also not wholly accurate: the driving force behind the LN2 volcano is essentially a big "gas piston," while most of the explosive power in an eruption comes from the rapid exolution of dissolved gasses in the lava (mostly CO2 and H2O). Erik Klemetti (Eruptions Blog) has some good information on the gas/pressure relationships governing volcanic eruptions. Simulating this rapid exolution of gas can easily be done by dropping in a few Mentos candies into a bottle of carbonated soda (Diet Coke works best, but I've had good success with cheaper generic diet colas, too).
And filming the reaction in slow motion (2,000 frames per second) makes them seem to last forever:
Slow-Mo Mentos and Soda Dubstep from Matt Kuchta on Vimeo.
Some diet soda plus a couple mentos, filmed at 2,000 frames per second and a little wobble bass added for effect.