Heya Echo,

With aircraft that have virtual interiors (i.e. Mel Rafi's 777 or Milton Shupe, Scott Thomas, and Joáo Paz's Howard 500) you can go to the virtual cockpit and use the Ctrl + <-- (Backspace) keys to "move" forwards and the Ctrl + Enter keys to "move" backwards.  Use the spacebar to "reset" the view back into the pilot's seat. 

Please note that this will only work with aircraft that are made by the authors to have virtual interiors.  Not all Add-On aircrafts will do this.

There are many Add-Ons which will offer fun ways to "drive" around the airport and look at the AI traffic around you.  Some of these are:

1998 Mercedes SL600 by Jessy Corrales

[url]http://www.simviation.com/fs2004misc3.htm[/url]

Classic Jaguar XK-120 by Alejandro Villa (available at the same page as above)

The ever-popular TG 5000 Airport Tractor by Hauger Fredo is a lot of fun.  Note that you can't actually "tow" an aircraft with it, however, it's lots of fun to tool around your airport with!

[url]http://www.simviation.com/fs2004misc7.htm[/url]

There may be some other airport vehicles that I haven't mentioned that others here may suggest.  But this will start you off in the right direction.

There is a downloadable program called Active Camera for FS 2004 by the AntiCyclonE group which will allow you to use different views of your aircraft.

[url]http://www.anticyclone.be/active.html[/url]

I have read mixed reviews on Active Camera.  Myself, I haven't tried it out, however, you may download it and see if it works for you.

Hopefully this answers your questions.

What you might also want to check is active camera, at www.activecamera.be. It allows you to walk around inside and outside your airplane and get all sorts of different views, it is payware though. You can move around the aircraft, like graycat said, using Ctrl+backspace and Ctrl+enter and it should work in all aircraft (whether they've got a vc or not, as far as I've tried, although not much will happen if the plane has no vc(you'll just be moving the viewpoint around in the air)). You can also use shift+backspace and shift+enter to go up and down (in vc) and ctrl+shift+backspace/enter to turn around (I think) if you haven't got a hat switch on your joystick.
Hope that helps,