Donana National Park

El Parque Nacional de Doñana is one of the hidden gems of Spanish nature, however it is not on the way to Jerez de la Frontera, you would have to double back up to Sevilla in order to cross the Guadalquivir.

Thank you Carlos for pointing that out!
We have four days in Jerez.
One to get there, settle, stroll, and maybe a tour of Pepe wine.
Another for the Horse Festival/Spring Fair .
Then two day trips, one to Arcos de la Frontera, and another to Cadez.
We thought on the way our on the 12th hit the park (thinking it was on the way).
Is it worth replacing one of our day trips for a day trip to park?
Is it a drive through or walking sort of park?

It's a bit of both driving and trekking around, the park is quite large. The gateway to Doñana is the Andalucian Cowboy town of El Rocío, which is worth a visit on its own.

Doñana has seen better days as the region is currently going through a serious drought and I've heard the wetlands are drying up and the animals are leaving, so I'm not sure how the experience will be nowadays.

As Carlos pointed out, unfortunately there are no crossings over to the park from the Jerez de la Frontera or Sanlucar de la Barrameda areas. Maybe visit the park en route from Sevilla to Jerez. Because the drive to the park is just over an hour you could drive over to the park and El Rocio, then head to Jerez. El Rocio to Jerez is just over a 2 hours drive. You’d have the better part of a day to visit it.

In Jerez, we never took the González Byass (Tío Pepe) tour, but did enjoy the Osborne and William & Humbert (Dry Sack) tours. I assume they are still offered.