I tend to look at areas that have less ranger patrols, and usually have some ant related stuff on my phone that I can pull up right away. If anyone asks, I just tell them exactly what I'm doing, and if they press for more I show them some webpages/pictures of my ants.
Fremont older doesn't have much from what I have seen. I have explored it quite a bit as I live nearby, but can never seem to find much there.
I would check some local, smaller parks. I have caught 100's of queens at a park right across the street. If it's surrounded by forest (oak seems to be best), and relatively undisturbed (i.e) not completely grass and concrete, it probably has some nice diversity. This park is local, and has so much diversity that within a 300 sq feet area, there are 5 species of Camponotus, hundreds of P. imparis nests, a handful of Formica colonies, Tetramorium (pretty much everywhere in my area, but a species nonetheless), Solenopsis molesta, and more.
I would really recommend checking out some smaller parks near you, that aren't preserves or county parks. They are much easier to ant in.