Right, your site is looking very slick now but I have to mention one point; you haven't cross browser tested it yet have you ?
Your CSS driven menu is all well and good and looks very nice in Internet Explorer but the code does not take alternative browsers into account and goes a little haywire in Firebird, older versions of NetScape and Opera on Windows and does similar tricks on IE running on my Apple Macs. Your links in the menu are only as long as each word or words that make up the link, so you get a staggered stepped effect on the menu (including the mouse rollover colour swaps) instead of the intended rectangular column of equal width links.
For many CSS driven menus to work over different browsers you need to do quite a bit of browser specific code hacks which is where a lot of people trip up when coding CSS. Without getting into a big lecture on the topic, different browsers interpret code in different ways and CSS is no different, so you need to include some browser specific code into the underlying CSS for all browsers to render it properly.
I can't think of any solution off the top off my head that you could implement that isn't going to be either horribly difficult for a non-programmer to deal with or overly expensive to buy.
No insult intended there, but I am assuming that you don't program Perl of PHP ?
PHP/MySQL and even CGI could all be used, but the third party solutions are all quite expensive and creating your own would be a huge project in itself, especially if you need it to hook into existing systems used by other companies.
Being an old fashioned soul, I would also suggest that you'd need multiple MySQL database support from your web host.
I normally keep a user authentication database (username and password login system) completely separate from online trading/catalogue/booking systems.
The information from the one allows interaction with the contents of the others, but I never include the authentication database into stock control or whatever. For any form of adequate user security you also need to get into implementing password encryption using the built in PHP crypt() or MySQL PASSWORD() functions, setting up user sessions and all kinds of other geeky things that only people like me get off on.
Perhaps you could be a bit more specific in the systems requirements one of us could come up with a solution that is a little less hopeless-sounding than my above rant.
Who will use the system ? Who will administrate it ? Will users be required to generate unique (and therefore identifiable) usernames and passwords for return visits or will this be a more open reservation system intended for one off use ? Etc, etc, etc...
Knowing what you need is more than half the battle in reaching your goal in this situation and the above are just some of the many questions that any web application programmer or IT consultant would ask in trying to come up with a solution to your requirements.
Come back with more and who knows ?