From the Altar Trail, you can see the Sea of Cortez and from the top of the trail, "Picacho del Diablo" which is the highest peak in Baja California.
To get to San Pedro Martir National Forest, you'll take Highway 1 down towards Ensenada taking the San Telmo exit to the "Sierra San Pedro Martir" sign. It is paved all the way so that even a compact car can go. There are campgrounds and cabins to stay a night.
The Altar Trail is better to start at sunrise to stay away from the heat. It will take from 3 to 4 hours roundtrip. You may want to bring trekking poles if you them as there are some steep parts to the trail.. Between the campgrounds and the trail, you'll find a valley for night photography to shoot the Milky Way since there is no light pollution.
You will also want to go to the Observatory (by car) in the morning to have a free tour. Autumn is one of the best times of the year to see the gold pines and maples.