Get MSR impact shorts. I got mine from
The pads aren't hard plastic, just foam. They're really thin, so you hardly notice they're there. They also act as compression shorts (great for not pulling your groin.) The only bad thing is that, because they're made for motocross, they have a huge ass pad, but it cuts out easily. They won't help you on huge bails, but they're enough for regular bumps and bruises.