I am partial to Caswell State park in Ripon. its about 15 minutes north of Modesto on 99. Of course you could alwasy go to one of the reservoirs nearby- Modesto or Woodward. Caswell is on the Stanislaus river and it has nice cozy little sites that are tucked into the greenery. If i remmeber right, it something like 20 dollars a night but there are showers and the like.