You don't need to buy an annual PPass membership if you don't want to. The Continental Presidents Clubs offer day-pass rates for entry that you can simply pay at the front desk ($45). Also, you can check in the Continental forum in the sticky thread for P Club meet ups. Someone might be able to get you in for free.

And if you are buying the ticket with the long layover because the shorter ones are cheaper you can likely fly standby on the earlier CO flights for free by requesting the change at IAH.