Out of your two ideas, I think that the text messaging one is more unique and not really being done yet. I've seen the Santa letters a lot recently so I think you would have more competition. Conversely, the Santa letters are certainly much easier than the texting idea. For example, who are the text going to come from? Are you going to do the texts? How are you going to get paid? What's the guarantee that the restaurant/salon doesn't just take your idea and cut you out? With the Santa letters it's easy - they pay you to write the letter.

Additionally, are there any regulations regarding mass texting?

Best of luck! :)

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