So you saw that portal was free on steam till the 24th. Thats great and then you downloaded it, and beat it in two hours. Now what? Well fortunately there is a map pack* that I will tell you how to download and install. This map pack is actually based off of the maps from portal: flash (a flash version of the game that serves as a prequel to the original portal).
Step 1: Go here and download http://portalmaps.wecreatestuff.com/...fv_mappack.rar
Step 2: Unzip the whole file into a new file on your desktop called portal maps (or anything you want)
Step 3: You could read the readme on how to install it, or since you are already here you can follow this
Step 4: copy the entire folder and place it in: C:\Program Files\Steam\steamapps\youraccountnamehere\portal\portal So basically you go to your program files, find steam, then steam apps, then your account name, then portal, and then portal again.
Step 5: Click replace all if necessary
Step 6: Load portal, then click bonus maps, then load the maps.
Step 7: Enjoy
If you run into any problems please reply back and I will try to address your issues and help you out.
If you are having trouble beating the game here is a link to some guides** on how to beat it: Portal the Flash Version Mappack: Walkthrough - We Create Stuff - Communites
* = I didn't make this. This is just a guide on how to get the maps and install them, and to bring your attention to the fact that there are more maps.
** = I take no credit for this either, I just merely found it, and found it helpful.