Lake Hodges, San Diego, CA, USA
Beautiful lake in EscondidoYou just see dry, desert scrub landscape, and then this beautiful blue gem appears! Lake Hodges is a reservoir, so you aren't allowed to swim in it (human waste particles are too small for the filtration system). But you are allowed boating and fishing privileges.
There's a trail that encircles the lake, and you can hike or mountain bike here. Recently, the city built a unique suspension bridge over a portion of the lake, where you can walk or bike out over the water- http://www.gostructural.com/magazine-article-gostructural.com-6-2010-dramatic_bridge_provides_a_natural_crossing-7918.html In the summer, the water is dried up under the bridge.
Again, since this is farther east in San Diego County, I don't recommend hiking out here in the summer or fall. But locals usually are used to the heat. Remember to bring plenty of water!