Fiesta runs in April, if you can go in April.
I have been to all Mi Tierra in Market square, Casa Rio on the Riverwalk, and La Fonda on Main. By far La Fonda's food is better, the best we have had in San Antonio so far. Mi Tierra is open 24 hours and a has fun, festive decor. The food is not the best, but not bad at all. Good choice for late night dinner. Casa Rio is good to enjoy the Riverwalk atmosphere. Just average food, but great location.
We like Natural Bridge Caverns. Amazing caves. There is also a safari next-door to it. Feeding zebras is fun! We enjoyed the botanical garden our last trip, but that may be because they had an event going on. It could be a little boring if you are not into gardens. We do like SA zoo. If you love birds, you get to see many and there is a feeding area too (lory landing).