a) arrange with Ikea to pick it up (at their expense), they might even let you keep it if it is easier or cheaper for them.
b) keep it and use a stolen mattress. It was delivered by mistake, if you don't try to return it and you keep it you are keeping something you or someone else hasn't paid for. If you sent something to someone incorrectly you would appreciate it if they tried to get it back to you.
c) Decide you want it and arrange to pay for it and keep it.
d) use it as a trampoline.
Anyone have any other suggestions?