If you have an item that you need to plane down, but the wood is too large for your planer (if you even have one), consider using your router as a planer to get the job done. Take a look at the image in this post and you can see how relatively simple this method is. This is a perfect way to smooth a large slab table that is is rather trendy today. Obviously, you can simply adjust the size of your jig to handle your lumber piece. If you are doing something rather wide, you should consider adding side pieces to your jig so that there isn’t flex in the cross member. Just the weight of your router across 30″ would be enough to cause quite a bit of deflection, not to mention the deflection caused when you start reaching across your piece and white knuckling it.
Good luck building this jig. It is a nice (and cheap) alternative to a planer.