Do you have a bookshelf stereo system in your room? Because if you do, there's always a huge compartment meant to hold a bunch of 'D' cells in the back. You could easily pack your juice and several darts in there. Also, if your home has central air...you could hide it in part of the duct. Remove the register, and slide your hand into the duct. If it's level after the initial bend, then you won't have to worry about things sliding away out of reach. It's a perfect spot to hide things.
Oh, and I strongly advise against the computer as a place to hide your juice. When a computer is in use for any significant amount of time, it can get damn hot in the tower. High temperatures can denature the active ingredients in a lot of drugs.