You just need to enable captive portal, setup your firewall rules accordingly, and ensure layer 2 isolation. To separate the guest network from the hotel's internal network, most commonly use VLANs, or in larger hotels, completely separate physical networks (and at times a separate firewall entirely on the hotel internal network, though that's not strictly necessary, some hotels require running that way as policy).
Detailed info in http://pfsense.org/book on captive portal and VLANs in general.
We're very experienced with these kinds of networks (several hotel Internet providers use a rebranded pfSense for their captive portal), would be glad to assist via commercial support, link in my signature.
No, our captive portal section has no relation to Untangle. :P They're late to that game, we've had that capability for many years longer and I'd estimate we have nearly as many installs running captive portal as they have total installs.