I'm glad you're introducing your students to programming like this! ^.^
Like Kaleb has shown immensely (xD), TI-BASIC is wonderful to use for simple formulas and equations through the use of Input and Prompt (btw Kaleb, if you want to optimize your programs, use Prompt A, B, C instead. ;P).
If you want to expand a little through, the classic random number guessing game is a good path. The calculator generates a random number, then it is the user's goal to guess the number. If the number inputted is too high, the calculator responds with a message like "TOO HIGH." If it's too low, "TOO LOW", etc, until the user guesses the correct number.
It's pretty easy to do, and helps them think logically to solve problems. Also, there's a TI-BASIC guide here which has lots of information about TI-BASIC, as well as some challenges to do too. ^.^