Ian wrote:
Will want to hire bikes for a few days while on a US holiday. If I do, will
I be charged lots for a helmet and for other extras like repair kit/spare
tube, so its worth bringing them on the plane from home in the UK?

Can anyone answer?

It really varies by vendor. Most believe people should ride with helmets
and include them in the rate, others may charge extra. We may be able to
help with names of shops who rent bikes if you give us an idea of where
you're visiting. Hope your holiday is enjoyable.