I used to live in Santee, but that was ~20 years ago and I have not been back in 15 years. It had changed alot in that time, I can't imagine what it looks like now.
A buddy took me out a few times but it was to an area I am guessing NE of Santee. I know we were into horse country. It was very nice and alot of options to make rides varying lengths. Around Balboa Park was nice, but it was not very big. La Jolla was nice, but hilly. I always liked the ride through Mission Trails Park but it was only about 1 mile long and no options to make loops through the park. I never got out to Lake Jennings Park, but maybe that is an option. Sorry, it has just been too long for me to remember, plus it may no longer be useful.
A good option other than this board would be a good local bike shop and even better would be a local bike club. I was only there temporarily so I did not join any to give you a club name.