Used to live in Cali and went to pismo 3 times last summer.
There is only 1 company that will rent RZRs and 2 seater buggies. Company is called sunbuggy and you can go to their website to see pricing and all that good stuff. There's 3 or 4 other companies that do rentals out there and on the beach but I suggest always reserving a camping spot ahead of time because they will turn you away if they are full and they always get full during the summer. They will let you day ride though.
The dunes aren't that big, there's tons of people out there and lots of noobs that don't know how to ride so it can get kinda sketchy with those people. If you get back there far enough, it's better as everyone lingers around the kinda flatter part of the dunes.
There aren't any dunes big enough to slide on.
I know I probably made it seem horrible but it's honestly a beautiful place and it's tons of fun. The camps are closer together at the front because so much of the park is closed off.
We are going in July if that tells ya anything lol