You'll get a fairly wide range of answers once you get beyond the "kit" lens, fix focal length fast lens (ie: 30mm f/1.4, 50mm f/1.4, 50mm f/1.8) and the long telephoto (ie: 70-300mm).
Most people will easily get away with 3 lenses. Most people who don't come to this board will use just the kit lens and maybe a long telephoto.
Many people that DO come here will go with upwards of 7 lenses.
On my trip in a few weeks I'll have my "walk-around" lens (currently an 18-135mm), 30mm f/1.4, 70-300mm VR, 10-20mm, and probably my 90mm f/2.8 in case I get in a MACRO mode.
For fireworks, you can easly get away with your "kit" lens if you use a tripod and a remote shutter release. This is also true of getting any other night shots of the castle, hat, SSE, Tomorrowland, World Showcase, etc... For the Safari a long telephoto combined with using a high ISO should do you fine.
For indoor rides you'll need a lens with a wide aperture (ie: f/1.8 or f/1.4). This is where the 50mm f/1.8's are recommended (they are very inexpensive in lens land for Canon and Nikon). Biggest drawback is the long focal length on a crop dSLR. Another recommended lens is the Sigma 30mm f/1.4. Lots of the regulars here have one and have had great success with using it (both at Disney and at home and other travels). Its pretty sharp and on a crop dSLR it has a great focal length (equilivent to what the human eye see's, or very close to what a full frame SLR's field of view would be with the 50mm).
At lot of the decision that goes behind what lens to take will depend on your shooting style. Some people only go with fixed focal length lenses. A few others like to shoot a lot with fish eye lenses. A few others prefer to shoot at the long end of telephotos.
