In my suburban high school social world, before cell phones:

• Loitering around shopping malls
• Showing up to the movie theater an hour or two before the movie we all agreed to watch together, and then an hour or two after.
• Parks and playgrounds if the weather was ok
• Cafes, diners, restaurants where lingering was tolerated, even for those who didn't order anything.
• Local sporting events (usually high school or rec league teams).

A lot of these still exist in some degree, but the loss of malls and movie theaters has really put a damper on things, and the rise of cell phones has deemphasized the need for in person interactions with peers (rather than following an influencer on social media).