Staining a Screened-In Porch

Our screened-in porch had been painted by the previous home owners. We just finished sanding off the old paint and are left with bare plywood. It's not pressure treated, just plain 1/2" plywood sheets nailed to the decking underneath. What would be a good option to finish the wood? Stain? Paint again (don't really want to do that)? Something else?

As mentioned, it's a screened in porch so the floor will get wet depending on which way the wind is blowing when it rains. We picked up some Olympic Elite Semi-Transparent but haven't applied it yet as we are having to deal with removing the paint from the nail dimples. Which brings me to my next question. What's a good filler so the filler doesn't stand out if we decide to apply a stain?

Thanks.