It's free to get into Pacific Park; rides are a few $s apiece. You can go to the beach there, but the water is usually pretty cold (that's CA beaches in general, though...if you're going in the summer, the water should be a comfortable temperature). It's free to go to the beach. As of last summer (that's when I was last there), there was a free parking lot but it's really, really difficult to find parking there...I suggest parking in one of the pay lots that you'll see around. That usually runs $5-$15, depending on where you park, what time of the year it is, and how crowded it is. My favorite thing to do there is go on the big ferris wheel...there aren't many rides (especially not compared to the pier amusement parks on the east coast...unless you also go to the beach, it's not really an all-day activity.) Also, you can walk the boardwalk and sidewalks to Venice Beach. It's a long but pleasant and scenic walk. When I go, that's usually what I do, then it's more of an all-day activity. From downtown L.A., it's about 15 miles to Santa Monica...not that far, but I've had the drive take a couple of hours when there was traffic

Also, you will probably want to be out of there before it gets dark...it IS a fun place to go, but it can get kind of "iffy" at night. (That's coming from me, an L.A.-native-until-a-couple-of-years-ago, so others might find it either "scary" or "no big deal", depending...YMMV

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Here are some links; hope they help with your trip planning
http://www.santamonicapier.org/
http://www.pacpark.com