Page full of Microsoft official Windows 7 Direct Download Links (Heidoc.net) for .iso's to burn your own DVD's to try out all the flavours you want before any activations. (you can save your oem activation for when you have made your mind up) Scroll down to the English USA section and get ones with SP1 U (media refresh) as they have sp1 on them. You will need a program like the free 'Imgburn' to burn an .iso to DVD.
Entering a product key during installation isn't mandatory. Just click the "Next" button without entering anything, and the Windows 7 Trial will be installed.