How to provide free wifi without leaving WAN vulnerable

Hi, I am trying to help out my Town, they have a wireless access point with no security, they want to provide free wifi for visitors at town hall. This leave their LAN vulnerable, what would they need to do to provide a reasonable amount of security to protect their LAN from wireless users but let the wireless users access the internet.

The have a cable modem connection with a Netgear FVS-318 Fiewall Router and a linksys WAP54G AP

Thanks