I left SD just over a year ago and grew up there, 55 years in San Diego County. Right now the best place to find something not so pushy/shovy "San Diego style" is Fallbrook. Beautiful wooded community and there are some more rustic parks. From Mission Valley I-8 go north on the I-15 and it's 30-45 minutes so if you are working in downtown not so close. There is also Escondido but that can be a rough area. Stay out of East San Diego and National City. IB is great but pretty locked up nowadays.
The other place would be El Cajon. The El Cajon Valley proper can be a rough area but go east to Los Coches Road northerly down the hill and turn right on what they call "Old Hwy 8" or "Business 8." Paved country road and lots of family parks out there.
FYI, Santee Lakes sucks! If you are totally upwardly mobile and don't mind taking out a mortgage to stay for a week or two you might like it. Very crowded and typical California me first rules and regs through the roof. The lakes are beautiful but the management not so much!!!
If you really want rural country, keep going east on I-8. Check out Alpine or keep going to Pine Valley.(really beautiful!)
In Pine Valley check out Boulder Oaks. Also a lovely place is Lake Morena in the Pine Valley area. I know most every street in SD County. Owned a large swimming pool route so on the road a lot!
And then there is Boulevard. Any farther and you have the Laguna Mountains and then down into the desert to El Centro etc.
Oh and one more thing: Mission Bay is of course off the scale beautiful but going to be very pricey! Campground on the Bay, Fiesta Island and Torry Pines north up the coast! All classic SD!
For day use go to Lake Murray in La Mesa and Lake Jennings in East County!