Well, you picked a reasonably good dive op.
(I have loved diving with LU for years; you will have a good time; give my regards to Jeremy, unless I will actually be with you on the boat!). I'm usually wiped after my dives due to length and therefore, my lowered core temperature. So I USUALLY come in the door, rinse my camera and computer, check my email and then hit the couch for about an hour or two. Then I start thinking about where I am going to go for supper and then I hit the sack early if I am diving the next day as 7:30 comes around quickly! I know...I am not any fun at all.
Not sure where you are staying but sitting in a local cantina in town where they bring you all kinds of free snacks is a great way to enjoy local colour and eat inexpensively. Jeremy knows all my favourites and can direct you accordingly, if that is the sort of thing that floats your boat.
If you have a car, you might head over to the east side for some sun, sand and suds, picking up a rotisserie chicken for a late lunch/early dinner. Afternoon movies at the theatre behind Chedraui are stupid-cheap and there is a bowling alley across the street that many enjoy. If you are athletically inclined, you can buy day passes to the local gym on 11th, just off Melgar...it is a VERY nice gym.
I'd stay out of the main downtown area in the afternoon until the cruisers head back to the mothership...although sitting at Thirsty Cougar on Melgar and watching the world walk by IS entertaining. Some people in other threads have enjoyed chocolate or tequila tours for an afternoon. And there is always San Gervasio ruins.
If you are at ALL inclined to horses and want a horse fix, there is a horse sanctuary on the island owned by a friend. I spend an entire day there helping her out but if you just wanted to come by and see the horses, make a donation, feed them carrots and apples, or groom a horse or two, that is another possibility. If that is of interest, drop me a PM and I can send you details.
I'm sure others will chime in.