I agree with g-funk, if you're going to go over annadel way, take her around howarth park/spring lake until she's ready for a little more adventurous trails. There's a little bumpy single track around the south spring lake dam, and some more around the boat ramp.
For beginners, I highly recommend Foothills Regional Park in Windsor. It's where my then gf, now wife, learned to ride. They have a very nice selection of trails for most all skill levels in a very compact area. Don't expect any epic rides there, but the trails range from REALLY smooth fire roads to pretty techy and steep single track. It's sweet because after you've been around it once, and that doesn't take long, you can say, "OK, you go that way, and I'll meet you down there". Then she can ride a nice, smooth trail, while you go get a quick, super intense hill climb, followed by a quick, but very exciting downhill; and then the trails merge again. I used to live right down the street, and I'd grab a ride before and after work almost every day in the summer.
It's on Hembree Ln, north of Old Redwood HWY, before Hembree turns into Arata.
Here's a link:
http://www.sonoma-county.org/parks/pdf/park_maps/FoothillMap.pdf
Alta Vista and Ravine are where the funk is at.
Good luck!
