I think I know the answer, but I'll ask anyway, cuz I value opinions from those who've been there, done that. We'd like to see the horse show in Jerez and take a sherry tour. Is it anywhere near feasible or sensible to try to cram that all into one day as a day trip from Seville? I'm aware the horse shows are only at noon on particular days. Or should we plan on an overnight in Jerez? If "overnight" any thoughts on where? "We" is me and my DW, in our 60s experienced European travelers, with poquito Espanol and Spain Passes for train travel so travel expenses are not a factor.
I think you could do both in a day trip if you don't go to Cadiz (per your other post). In May the sun won't set until after 9pm so you can pack a lot into a day! I don't know what else there is to do/see in Jerez besides the horse show and sherry tasting, maybe somebody will jump in with suggestions. Be aware that the Feria de Jerez is held in mid-May and the town gets very crowded at that time, and the trains from Sevilla can sell out (so hotels probably get fully booked too)probably more of a problem on the weekend. They do have a special nighttime horse show on the Saturday of the Feria although I think that tickets sell out quickly. The train schedule may be the limiting factor, although it looks like there are lots of departures if you don't mind the slightly less-comfortable Media Distancia trains. The bus is another good option in that area.
Tom, last June we did the horse show, and then drove to the hill town of Arcos, had lunch, leisurely explored Arcos and drove back to Seville. There was no need to rush, or get up "at the crack of dawn. " or drive back in the dark. We are in our late 50's and 60"s and found that using a rental car gave us lots of flexibility. We used the plane to move from BCN to Granada, and then used a rental car. We could have just as easy used the train for much of the trip, but the car gave us the flexibility to explore the hill towns, and things like the Pileta caves outside of Ronda.