The usual method is to mix vinegar and red food coloring into a cup or bottle, then when you want it to erupt, you add baking soda. The hard part (though its not that hard) of this is making the actual volcano (using plaster of paris, or similar) to contain a cup/bottle for the vinegar. And remember that this can make quite a mess, so put something underneath it.
Building volcanos is fun! Just a few simple ingredients will cause an eruption, but the fun part comes in how creative you get building the actual volcano. Adding 1 tablespoon of liquid dishwashing soap, 1 tablespoon of baking soda, and a few drops of red food coloring into a 2 liter plastic bottle, then pouring 1/4 cup of white vinegar in will give you an eruption.
Build a base for the volcano with plywood. Just get a piece of plywood bigger than the size of the volcano. Put the plastic bottle in the middle and use paper mache to build out the volcano around the bottle. Make sure you build right up to the opening in the bottle. Make ridges and valleys for the lava to flow through. Once it's dry paint the paper mache to look rocky. Add in some trees or houses at the base for the lava to flow over. Add the soap, baking soda, and food coloring into the bottle. Once you're ready pour in the vinegar and you have your very own Mt. Vesuvius.
I'd suggest trying this out for the first time outside, where your lava won't get all over the carpet.
small drink bottle
60 ml water
1 tablespoon baking soda
1/4 cup vinegar
orange food colouring
few drops of detergent
small square of tissue.
Making the eruption:
Place the water, soap, food colouring and vinegar in the drink bottle. Wrap the baking soda in the tissue and drop into the bottle. The volcano model will then erupt.
For extra realism:
The volcano can be made more realistic by enclosing the structure in home made play dough. Mix 6 cups of flour, 2 cups of salt, 4 tablespoons of cooking oil, and 2 cups of water in a large bowl.
Mix the ingredients by hand until smooth and firm. Build up the mixture around the drink bottle to create the mountain. Lava channels and vegetation can be built around the volcano.