Guest WiFi captive portal solutions

I'm setting up a network for a small restaurant that wants to offer guest WiFi access as a selling point, and use that to drive traffic to their social media sites.

I was wondering what kind of solutions are available for this task?

I'm currently testing with the Facebook WiFi option in Ubiquiti's Unifi line. I liked this because it gave customers the option of checking-in on facebook, which takes away the need to post up signs with the password or require staff to remember the password. But FB WiFi has a stipulation that guests can access the internet even without checking in, which kinda defeats the purpose.

There are other options using UniFi, but it got me thinking about what others are using.

I prefer the free/opensource route, or at least something that doesn't cost hundreds, or require bespoke hardware. The network solution at this place is simple with an ERX, Unifi AP, and an x86 mini PC running Debian.

Thank you.