It depends upon what you like doing. This is not a botanical garden in the sense of strolling through a place that has featured botanical displays, such as the New York Botanical Garden. This is more a garden that could be used for a round of golf, if they allowed it. It has large swaths of grass, with select plants located throughout. It left me with the impression of being more of a public park to relax or picnic in, rather than a place of botanical displays. The plants that are there are established specimen plants, such as cycads and banyan trees. There are also bridges over water features. So think of it more resembling a public park rather than a botanical garden with floral displays. And like a public park, there is no cost.

I don't know about how you book events at the garden, but I do know you can book pavilions at the Panewa zoo just South of Hilo.

It's an open Parkland. Free entry.

No info on holding weddings in the park. Suggest contacting the Parks and Rec department at the Hilo courthouse. I was married in a fifteen minute Buddhist ceremony at the meishoin in Hilo. The minister was Junshin Miazaki and the ceremony was truly beautiful. You don't have to be Buddhist and the suggested donation is 400 dollars. This would be an indoor event, so if you're looking for Hawaii scenery, your idea of using the park would be better.

Not a good idea as it does not have adequate lighting in the evening and you could turn an ankle as the surface is uneven.

There is a lovely restaurant next door, Hilo Bay Cafe, which may do functions. Lovely view of the bay, delicious food, and walking distance of the gardens.