I co-founded Pigeon SMS (https://pigeonsms.com) with my dad just a couple of years ago and we reached profitability just a few months after founding.

We provide business texting services to businesses of all sizes. Some of our customers include a city, vet clinic, glass shop, screen printing firm, boutique retailers, and on and on. Really any business can benefit from adding texting to their communications.

My advice: Pick something that will keep you interested and motivated to come back to when life gets busy.