I'm really happy when I have good music on and can settle into a creative niche, where what I'm doing (writing, art, etc.) comes out really nicely, and it seems like I could keep going for a long time.
That's even better when I know I don't have anything to do anytime soon.
And, also, settling into my bed thinking about sleep is awesome too (Rei described it pretty well).
Makes me feel all warm and fluffy just thinking about it.