I've been building things in this game every chance I get. Its awesome building a little fort to protect yourself from zombies, or barricading the combine out of a room.
Also, anyone get up the part where you first get friendly turrets? I kept all the turrets, and painstakingly dragged them along untill I amassed seven turrets. Its like a firing squad of miniguns when you aim them all at a single door.
Its 7:30 am, and I still plan on playing till about noon (sleep is for the weak willed, and the healthy). As soon as the SP is beat HL2 is going to collect dust until some multiplayer mods come out.
Anything but more Counter-Strike I'd play. Even if it was vanilla deathmatch I'd jump at the chance to play it. With the right group of people, you could organize special game modes, like grav gun wars.
EDIT: Oh, and this sums up the first few minutes of the game (at least for me).