I do not believe you have to pay for bench areas, however, the pavilions do have a cost. I suggest you call 311 to inquire.
If you are just going to the park for your party, go early and pick tables together. By the train station there are several tables close together under trees. This is free, but get there early especially if you are wanting the weekend. There are structures that can be used, but you may need to reserve and a cost may be associated. Not sure on that or who to call regarding reservations.
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