There are lots of cool things to do on the Peninsula from Millbrae. My old stomping grounds.
Crystal Springs Reservoir, just west of Millbrae along the San Andreas Fault line, is very scenic with a paved hike & bike lane, benches with views, etc. You can start at either end, though the northern part is much steeper than from the south entrance. Crystal Springs Road is a good way to get to the south end.
You can head to the coast. Go north up Skyline Blvd and turn left on Sharp Park Road or if you'd rather not go down a twisty, steep road, continue on and look for the signs for Hwy 1 south, go across Devil's Slide and the views are great. When you get near the town of Moss Beach look for the signs for the Fitzgerald Marine Preserve on the west side of the highway - gorgeous place and if the tide is out you can see lots of cool sea critters in the tide pools. This should show up on Google Maps.
I recommend lunch at Barbara's Fish Trap in Princeton by the Sea. Turn west on the road to the Princeton Marina, but go straight instead of left into the marina proper (though it can be interesting, too.) Barbara's is just a little shack on the Half Moon Bay side of the road. There will be lots of folks waiting, but it is worth the wait.
Lots of beauty all along the San Mateo Coast.
If you're in the mood for SF Bay, go on down Millbrae Avenue to Bayfront Park and walk south along the shore. There are nice restaurants along there, too.
