One of the things I did to start the gold rush was to hide gold-covered chocolate coins in the yard outside my room. Then I told one student at recess that there was a prize outside the door and after they discovered what was there, they could tell another student or keep all of them for themselves. It usually caused quite a "gold rush" of its own! It was a great way to show how things snowballed and how the word spread.
Break it into subjects and brainstorm how you can relate information to that subject.
Math - distances travelled, amount of money made, price per weight/size
Social studies - timeline, where people travelled from, what happened to the families they left
Science - how does gold form,
LA - make brochures to encourage people to move, write letters to family back home from the perspective of someone who followed the gold rush
If all else fails, find a lot of good trade books and develop ideas from there. Children of the Gold Rush by Claire Rudolf Murphy and Jane G. Haigh The Bite of the Gold Bug: A Story of the Alaskan Gold Rush by Barthe DeClements and Dan Andreasen
For resources, try:
I like to do a literature unit based on the ethnic groups that traveled to CA. I use books like Coolies, Hurry Freedom, etc (there are lots to choose from). I also use as a read aloud By The Great Horn Spoon. It is a great read, really funny, and the kids love it.