Add Continental Speed Rides to the tires rumrunn6 suggested. Excellent all rounder 700x42 tires (nominal, actually closer to 38 wide) for pavement and modest trails. I run 'em soft for rough roads, around 50 psi, and up to 60-65 psi for better pavement. Very easy on my damaged neck and back with my rigid fork mountain bike. I tried three sets of most-terrain tires before settling on the Conti Speed Rides and plan to wear 'em out and get 'em again.
I thought about trying wider tires too for a cushy ride. No problem with the frame, but the front derailer cable clamp bolt would rub with anything wider. Only way I could run wider tires with this mountain bike is to convert it to a double chain ring or possibly a single.
Unfortunately the 700x42 tires are too wide for the SKS fenders that came with the bike, but I may get some quick attach/detach splash guards for this bike and save the SKS fenders for a more general purpose bike.
Another reason I probably won't add regular fenders to the mountain bike is a few times riding off road I've snagged some debris -- small fallen tree limbs, large bits of dried brush with tough stems -- that might have tangled a fender and tossed me overboard.