Take a taxi to OSJ (about $12) Store your bags at audiogilla (or something like that) near the Wyndham across from the carnival pier. It's $1 per bag. Walk around OSJ. Go up to fort morro. Have lunch. It's lovely.
You can also book a tour through the cruise line.
If you are leaving on Sunday, I'd get to the AP no later than 3 pm. It will take a while to get through TSA -- they don't have enough staff / scanners to get the crowds through.