idle rainy day pondering - cheapest way to have a phone number + a little data for a backup iPhone?

[clay]

I have a now-unneeded iPhone 5s (Verizon, GSM sim unlocked). Thinking of keeping it around, both as a backup in case my main phone dies. Also considering keeping it to take on bike rides/runs as a audio listening device, fitness tracker/GPS device, and emergency call/text device. To that end, for it to be truly useful, it needs, at a minimum, a small amount of cell data. I could get a google voice number and route all calls over the cell data, or could sign up with a super cheap cell plan. I like the idea of having a device that I can take with me that doesn't ultimately matter whether it gets wet, breaks, gets lost or is stolen.

Wondering if there is a way to get data or maybe data+calls/texts for under $5/month? Free option would be a no-brainer. $1-$2/month probably still a strong "yes". But even $5/month seems like more cost than it would be worth to me.

any tips?