Depending on where you are riding, there may be restrictions on the type of vehicle you can use. California has red and green sticker registration. Green sticker is year round, along with plated dual sport bikes. I can't recall ever seeing plated ATV's or side by sides in this state.
This link is a PDF for the red sticker riding schedule:
click hereMost riders I know put their bikes/quads/toys away for the summer. Any of the desert areas will be to hot. The mountain riding is better for the summer, but the locations are few and further away. Look to Big Bear, Mammoth, Kennedy Meadows or Pismo(Oceano Dunes).