It's a steep sided gorge with some fascinating geology, a very cold river with rocky and sandy places to park yourself. Sun and shade possible if you can find a spot.
Go early - the crowds always build up - and use the lift, not the hundreds of steps (why waste energy and get ever hotter?). Young children will find the water very cold (some don't seem to care!)... the stones underfoot rather uncomfortable and the water current rather too much in many places.
Take sandals/flip-flops because you'll want to wade through the waters and it's stony. Take swimming gear, towels etc. Several of us ended up wading up to our necks then swimming against the strong current.
There is one wardens watching the river with his whistle never far from his lips, but anything you do is at your own risk. The water is very cold (I guess 5-7 degrees, even in the hottest of Summer days), but you do get used to it. The current is strong, so don't get out of depth unless you're a very good swimmer.
Whatever others have said, we simply parked in the main car park. No one approached us for money. But no one had mentioned the entrance cost! It was 30 Euros for a family of 2 adults, 2 Children. Else 13 Euros per adult and 9? per child. Still, worth it if you stay for several hours.
The walk along the top of the gorge (heading north) did not provide any great views, though the very last view point gave us the nicest view of all. So I'd venture to the end of the trail to see it if I was there again.
Some excursions add this place to their itinerary but I can't see how you'd want to rush in and rush out that way (and several trips don't include the entrance fee in their costs).
Restaurant food was good... though drinks started at 2.5 euros (common practice) which is pretty steep. Toilets were functional (in that less-than-pristine touristy sort of way), staff helpful and the 4D video worth seeing. (ask when in English, coz they play both English and Italian versions)
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