My short list:
Wildlife - wife works as a rehabilitator and educator, so I have pretty much unlimited access to birds of prey - call your local nature center and ask if you can take some portraits, most centers I've been to love the exposure and provided you give them a copy for their own advertising/fundraising have never balked. Again, that may be due to my wife's position but it never hurts to ask. Same with anyone who works with wildlife - just this past weekend I went on a hunting trip with a local falconer and got some great shots of his Peregrine falcon and Harris hawk in flight and with a kill. He was ecstatic when I offered him photos and actually was actively trying to accommodate my photography.
Zoos - see above, wife also worked at the KC Zoo for several years. I have several hundred photos, many behind the scenes or extremely close, of a variety of animals. Even if you don't know anyone, many zoos also have photo clubs that sometimes get special access to exhibits.
Weather - honestly, sometimes getting up early or staying out late will provide you with some interesting shots in normally "boring" locations. The landscape lit only by a full moon, clouds being lit from below by a rising sun, dew hanging off blades of grass as the sun is just beginning to rise...all of these things that I've tried or would like to try shooting. Corny maybe but if it sounds fun, do it.
Museums and galleries - anywhere that will let you take a camera, have fun. Many sites do not allow flash, so pack a fast prime.
That's all I can think of - I'm hardly an expert and am not that good but I enjoy shooting and when I DO get a good shot, it's worth it.
-wes