Pick a topic: doors, windows, post boxes, street furniture, store windows, bicycles, funny signs, people, museums, boats, textures, and concentrate on it. You'll build up a set of images that won't all be individually stunning, but as a portfolio, it may turn out to be a very compelling set of images. It'll give you something to do, it'll keep your trigger finger exercised, and it might generate some creative juice.
Use different camera gear: The last trip my wife and I took, I left the DSLR at home. I've got a new Sony that fits in my pocket (barely), and its 24-70mm equivalent zoom range is perfect for street and travel photography. It was freeing not to have to carry a heavy bag and all that gear, and I never missed having a telephoto or super wide angle or external flash. With different camera gear, you approach different subjects in different ways.
Write blog entries: It gives you incentive both to find something interesting to shoot, and then share it. I've uploaded 5-6 different entries from that 2-week trip, each involving an entirely different topic, which I captured with the Sony camera. http://www.heiserhollow.net/