I went there this summer. Holy fuck that place is rough as fuck, but a lot of fun if you can shred everything/most of it. There's stories of lots of ppl sleeping around the park in tents (from the park you can see a tiny house where some electric/whatever thing is hiding from the elements, you could try to put the tent ON that, looked pretty chill when I went).
anyways there's another pretty big park nearby, Leioa, and next to it there's an empty lot of terrain just behind some high bushes, so a tent there is invisible (Ive slept there). there's also a pretty big park, trees and grass mostly, near the skatepark, but I'd much rather go for the wilderness Ive said.
I wouldnt sleep in a tent on the beach, since early in the morning theres this brigade of ppl coming and cleaning it. and dont just try to sleep in any "empty lot" (dunno how to call a square in which buildings havent been built yet). the wilderness is better, no one gives a flying fuck there. if you can afford some nights at a shelter, i have no experience on that, but they say its pretty cheap.
ill tell you more if i remember any helpful info from my trip.