I like the atmospheric feel to the last photo especially. You might want to investigate gardens as well, to get additional landscape design ideas.
i particularly like the water under the bridge shot. this kind of vibe will work great for your layout.
Very nice reference pictures. Maybe try taking some pictures at different times during the day to get different lighting effects? It might help to get some neat color combinations. You can't really see the sky in any of these. I think the sky can have a huge influence on the overall color scheme and mood of the environment.
I love golden gate park so much. Those are some great pics! the painting one is great.
I'd also focus on the flora that will be in the park. What's native to the geographical area? Are the plants drought tolerant, coniferous or disiduous?