The Crank Brothers M17 Multi-tool is an essential to carry in your seat bag. It has everything you need to fix a variety of issues like a broken chain or spokes. Even if you don't know how to use every tool, you're likely to find someone on the trail who does and can help get you back riding.
The best way to avoid flats in this day and age is to run tubeless tires. Lose your tubes – you won't regret it!
My favorite MTB ride in the valley is Cutthroat Lake/Cutthroat Pass. It’s a mellow 2 mile ride to the lake, followed by 4 miles of hard climbing to the pass with gorgeous views at the top. The downhill is a blast.