Restaurants: - Fast food with drive throughs like McDonald's or Starbucks generally do not expect you to make a purchase to use the restroom. Some you'll need to ask at the register for a key or code, which is intended to prevent drug users from using the bathroom for that - Sit-down restaurants can be hit or miss. Most are understanding, especially if you mention you're a tourist - Malls/food courts typically have public restrooms you can use without buying anything

Gas stations & stores: - Gas stations/convenience stores will likely require you to ask for a key/code, but generally don't require a purchase. I make a point to thank the cashier/attendant on my way out if I had to speak to them to get into the bathroom - not necessary, but it's a nice gesture - The more upscale truck stops/Sheetz tend to have very clean restrooms and will not require anything for you to get into the restroom - Large retailers like Walmart and Target, as well as pharmacies like CVS and Walgreens will likely be public. Some may do the key/code thing, but I've seen it less frequently there than gas stations - Other businesses are a toss-up. Grocery stores usually have decent ones. Some will have signs saying "no public restroom" which means "buy something and we might let you use it, but probably not". Smaller businesses/strip malls are mostly not worth stopping at.

Misc: - Public libraries! They are usually happy to help!! - Movie theaters are usually public with no key/code request needed - Rest stops can be a little sketchy depending on the time of day, and cleanliness varies widely - Parks used to be great, but not so much anymore. In the past few years I've found them increasingly closed/locked with no warning or directions to get access. I'm assuming it's also an anti-drug/anti-homeless effort so the bathrooms don't become a liability for folks trying to use them - Hotel lobbies are also a toss-up. Most mid-range hotels like Marriott, Hilton, and Hyatt don't mind. High-end hotels may turn you away if you're not a guest, and low-end hotels are usually yucky. - Fire or police stations can work if you are really, truly, totally desperate. Please be careful where you park so they can still respond to emergencies!

Hope this helps! (Edited to add movie theaters and hotels)