during my second year of university, me and my (lesbian) housemate tricked my friends into thinking we had hooked up. the best part about this particular plan (we thought) was that the two of us actually built it up progressively for a full month - we each deliberately told our most gossipy friend about it in a really secretive way at the beginning of march and waited for the rumour to spread, then spent the next several weeks playing through all these really elaborate, dramatic, complete fake little stories, including a "panic attack" i had about my sexuality and my housemate not knowing what to do because she really loved me and me getting mad and not speaking to her and her wanting to kick me out of the house and so on and so forth. then on april 1st we publicly revealed to everyone that the entire thing had been an act, at which point the phrase "hell hath no fury" becomes probably the most appropriate descriptor for our friends' reactions. totally worth it.
oh hey it just occurred to me how to save this lame thread! everybody share your april fool's pranks! duh.