All US airlines will sell a passenger day passes either online or as a walk-in. There's usually a discount if you buy the pass online, for example a one-time pass to the RCC is $39 online, but $50 if you buy at the club. However, the RCC agents take away your day pass when you use it, so it's not reusable all day like the PC passes (but if you show your PC pass at an RCC, they'll return it to you).
DL probably has the best lounges, CO is probably the best for access to the most lounges and their day pass policy.