The nspire has 3 types of programming languages available:
BASIC: very limited, useful for mathematical programs, but no graphical commands whatsoever.
Lua: simple and quite powerful language and much fun to work with, works on OS 3.0 and above. If you have questions about this you should poke jimbauwens, he's very fond of it and has helped me quite well before
C: if you hack you nspire using ndless you have the amazing power and speed of C to add to your arsenal.

Some useful websites:
http://www.ticalc.org
http://www.ndlessly.wordpress.com
http://www.inspired-lua.org

There is also ASM on the Nspire if you ever want to venture into low level stuff, but with C around, very few people (calc84maniac) still use it anymore.

Another alternative is creating NES, Game Boy, GBC and GBA ROM hacks and make sure they run in NESpire, gbc4nspire or gpSP-Nspire.