The day I run out of ideas is the day I'm pushing daisies
The one thing I love doing. Grab my netbook, bike out to some isolated place (usually a park, no cars or people), and brainstorm. Spend a couple hours, just relaxing, writing and drawing. By the time I get back, I have a clear goal of what needs to be done. Spend a couple more hours documenting a proper design with all the components roughly in place. After that, it's usually smooth sailing. Works amazing for solving and handling individual components too.
Sometimes it also helps to work backwards. When I do the oddball weekend adventure, I take a list of resources I currently have and think of a way to make a game out of it. One time I had a bunch of animated fish sprites and I just whipped together a simple fish game. Stuff like this is fun because you give yourself a short, fixed schedule so you expect to see results right away.