Cowsay is a classic, although it's a lot more tame than fireworks — it isn't even animated. You could take it up a notch with Ponysay.
On the animation side, asciiquarium, which uses the Term::Animation Perl module. Unfortunately, I can't find much else made with that.
I also discovered that the ncurses framework comes with a number of demo programs, including firework. It's not the most beautiful, but you could also adapt it to be more dramatic without much work. Perhaps also because it's not that great, the Linux distro I'm on doesn't build and package the demos, but it's really easy to download the source, untar, then change to the test directory and do the standard
./configure; make. (You probably already have ncurses on your system and so don't need to build the whole thing.)
You could also use
worm from the same demo collection. I find it oddly mesmerizing.