By "detectiveish", I assume you are interested in forensics and related topics. How about fingerprinting? You could fingerprint a whole family, and see if there are resemblances, or you could compare male versus female fingerprints. You could test different surfaces to see which hold fingerprints the best, or you could see what variables affect how well a surface holds prints; for instance, what is the effect of different temperatures on how well a drinking glass holds fingerprints?
You could investigate bite marks; have subjects bite down on a piece of a Styrofoam cup, and see if you can identify patterns -are females different from males? Do bite patterns run in families? Can you make a positive ID from a bite pattern?
You could make a photo ID gallery and test witnesses for how reliable memory is; you would make a spread of photos of random people (use people that you test group doesn't know). Then you show your "witnesses" a copy of one of the photos briefly, then maybe an hour later have them try to pick the correct photo out of the photo spread. Then repeat the process, but the second time wait a whole day between viewing the picture and picking the person out of the photo spread.
You can find other interesting project ideas here and here.