There’s a lot to see, depends what you’re most into though as it’s massive and you’re not going to see most of it in one visit. The Temperate House that was just refurbed is still a bit raw/fresh and also they’re really busy, but they’re cool buildings, the Palm House and the Princess of Wales glasshouse are probably better just now.
I like big trees so for me it’s usually about that and the lakeside walks and that. (usually find peacocks near the redwood grove which is pretty cool) Treetop Walkway is great for kids, freaks me out though 🙂
I’d definitely take a picnic, tbh the cafes just aren’t that good, never mind price. Good for icecreams and that. I’d also take proper wild bird food, because there are canada geese and I like to live dangerously.