My parents live in North County (Poway). So most of the riding I know of is going to start about 20 miles Northeast from where you'll be staying. But if you're talking about riding all the way to Jashua Tree, then it sounds like you probably won't care. Here are some fun places to go ride. I put in the destinations on the map and you can plan your route out to those places however you want. Or you can PM me with your phone number and I can give you some suggestions.
+Palomar Mountain
Map - very popular riding spot
+Borrego Springs
Map - the road that drops from Ranchita to Borrego Springs is fun, and it's fairly close to Palomar.
+There's a road called Highland Valley Road that goes from Escondido to Ramona. It's a fun road, but be very careful. Lots of rolling hills and blind corners.
+The road from Ramona to Julian is a nice ride. Make sure to stop at Dudley's Bakery in Santa Ysabel. Good stuff...
+I've heard the road that comes up off the 8 freeway to Mt Laguna is fun, but I've never been down there.
+There are lots of small backroads near Lake Henshaw (right by Palomar Mountain) that can be fun to go explore. Some are not maintained as well as the main roads though, so be aware. There's a road called Mesa Grande right by Lake Henshaw that is very nice, but there almost always leaves on the road.