1 day trip near Santo Domingo

Do you mean Rancho Campeche? It's less than an hour from Santo Domingo, off the road between San Crist?bal and Ban?.

Rancho Campeche

If you decide to go, contact them first to make sure they are open to the public that day and to let them know how many people are going if you want them to cook for you. They do nice simple Dominican style food with plenty of salad, vegetables and meat and vegetarian options for something like RD$400 per head. They sell beer and soft drinks. I think you can also take a picnic, but they discourage the use of disposable plates and cups. They have a pool, horseriding, cycling, and relaxing in hammocks, and the finca is large enough for people to take a long walk.

Another nice place is Cambita Garabito, also in San Crist?bal province, high up in the mountains, nice and cool air. You can eat at a good, fairly expensive restaurant called L'Aubergine.

I've seen photos of some beautiful rivers and waterfalls where you can take a picnic and swim in Monte Plata province, don't have the details to hand though.

Here you go with L'Aubergine:
Aubergine Restaurante en La Colonia, Cambita-Garabito
The scenery and temperature are worth the journey, but apart from eating at the restaurant there is not much you can do in the area, as far as I know.