Ad hoc wifi ticketing

I currently have Cisco APs throughout the office that run two SSIDs.  One is our internal network and uses RADIUS (IAS) to authenticate domain members and allow employees to connect to the network.

The other is a public network for use by clients and visiting personnel.  

These two are on separate VLANs.  Internal is obviously all access.  The public one is simply a direct pipe to the Internet.  No access to anything else.

I don't want to tamper with the internal one as it works exactly like I want it to.

The public one however I have issue with abuse by external people who are close enough to get the signal.  So I basically locked it up with a simple WEP key.  I provide this key as needed to people visiting on request.

Here's my question:

What I'd really like to do is have some sort of ticketing system whereby I can issue a key as needed that will expire after a predetermined amount of time.  Something similar to how a coffee shop sells wifi access.  Buy at the register, they issue a key, and it's good for the amount of time you paid for.

I'd like to do something like that here for our public wifi (but obviously without the "pay" aspect).

Comments and advice would be appreciated.  I'm looking for the simplest complete solution.

Another thought... if I could use a SQL database as authentication, I could write an ASP or PHP ticketing system for automated ticketing via an internal website.  Employees could issue wifi tickets to their visitors themselves.

Thanks.