The TI graphing calculators have a mode for writing your own programs. Typically this is done using TI-BASIC, but if you prefix the program with the AsmPrgm symbol, it will allow you to directly write Z80 opcodes as hex values. Combined with a TI-provided guide for making bcalls (calls into common routines that have already been written, like changing LCD settings or multiplying floats), it's not too difficult to write a short ASM program by hand.