This one is simple, and it is inspired by the movie Waking Life. Simply find a light switch while dreaming, and attempt to turn the lights on or off, depending on the current state of the light. According the movie, there are a couple of indicators that can tip you off to the fact that you are dreaming. The most common one that most people experience is having difficulty reading numbers and words. Digital watches may seem blurry for some people, other people have difficulty reading a poster, or a page of text. For me, when I have come across this -- the letters in the words tend to shift and blur, not only that, the meanings of the words scramble, so that a single word can mean five or six different things in the time it takes to read it. Usually, this is combined with a sense of frustration (probably because I love reading so much) and a sense of urgency, that what is being read is vital. After I realized this was happening, several years ago, I've trained myself to become aware of it while dreaming, and realizing that I cannot read something lets me become aware of the fact that I am dreaming.
Also according to the movie, there is the trick of the light switch. I have never noticed this before, and I cannot think of a specific example where I have even tried fiddling with the lights -- but while you are dreaming, it is impossible to control the level of light in your area. Technically, even if you do find a light switch, nothing should happen when you toggle it.
Just last night, armed with this knowledge, I think my brain is trying to rebel against it because I had a dream where I set about trying to increase the light. Presumably, since I couldn't find a light switch, I was setting fire to things. Oddly enough, despite the fire things stayed dark. So I suppose the first objective is a bit of curiosity on my part, has anyone managed to toggle or change the lighting in their dreams? If so, we'll have to discard this objective since it doesn't hold up to reality, and consider it creative license on the part of the movie. If not, then the challenge remains.