I recently ran into that problem, and used the old jointing sled trick for the planer. It is so painfully simple that it should just be called a hunk of wood and a hot glue gun. Literally, a piece of plywood and a hot glue gun...
Shop tools and jigs just don't get any simpler than this!
So what I did here was took a piece of 3/4" cabinet grade plywood that I verified to be dead flat with a known good straight edge, and cut it to size. In this case 48" long which is the factory width of the plywood sheet, and as long as the stock I will be working with. I cut it 13" wide (okay 12-15/16" wide) to be able to be slid through my 13" Ryobi planer.
Wide piece of walnut on the sled waiting to be hot glued and
run through the planer.
The method of mounting a workpiece to the sled is simple, pick your direction to mount the wood to the sled, grain orientation / etc.. and the hot glue the board to the sled. Then run it through the planer until it is flat.
The idea for the planer sled came from http://www.thewoodwhisperer.com/videos/episode-6-the-jointers-jumpin/ and details can be seen in his video at the 11:10 time index mark.
For those that are even more space cramped in their shops, I would highly recommend this planer sled, and using it double duty as a jointing sled for the table saw as shown in the Wood Whisperer video. I have seen the arguments ad nauseum about folks needing a jointer before a planer. And I will state it here clearly, and plainly. You don't necessarily NEED either. It is easier to get by without a jointer than it is a planer, but lacking either, or both is not an insurmountable hurdle. You just need to make some common sense jigs and get good at using them!