how to challenge a 5 year old

You don't..... You let the child be a child. Just make sure the child has toys to build with, invent with and of course pencils, pens and crayons and lots of books. I brought up a very bright children..one with a exceptionally IQ. I have bright grandchildren as well. children learn through play.

No "hot housing" unless the child chooses it. I worry for those children who are pushed.
Think of what does happen in say childrens football teams where the organisers feel they need to ban the parents from the touch line as the parents are so pushy and upset the children.

Oh dear - how I hate the word stretched in relation to bright children. What does this mean? Just answer their questions, follow their lead and above all else encourage them to have fun with their peers.

I have a GS of 8 whom his friends call The Prof, because he has a brain like a planet - but no-one is trying to "stretch" him or making a big deal out of it - school has him free-reading, and he works at his pace in each subject; and they encourage his myriad interests. His parents were determined that he would go to the village school and muck in, rather than the posh city private school - I am sure their decision is the right one. He is a great little chap; and always helpful to a classmate who is struggling.

Lots of books at home; and the ability to look stuff up on the net. He has a dyslexic mother, who always jokes that she has no idea where his brains came from.

Im not sure how you tell if a 5 year old is "gifted academically". There are plenty of children of that age with amazing vocabulary, sophisticated language skills, who can memorise all sorts of trivia or gifted with numbers etc. I agree with others above, don't try to hothouse - remember those programs about the tiger mother who forced her children to do music practice, learn loads of languages, do extra tuition etc for hours every night. They never got a chance to be children and I read some follow up where they refused to go along with the mother's schedule and one ended up making adverts and I can't remember what the other one did but it wasn't some high flying profession.

let her lead,I dont think hothousing kids is a good thing .If she is truly gifted it will show but remember many parents believe their kids are gifted and its not always the case...think the awful x factor auditions etc.

Usually they're specially bright in one area, and weak in another ( mostly social.)
From the very bright children that I've come across their main need is to be encouraged/helped to inter-react socially.
As well as that, reading. And learn to play a musical instrument. Or taking apart old machines, electric gadgets etc.
I'm against making a big thing about it. Don't treat them as if they're superior/ different from others.

Reading to her, her reading to you.

Plenty of food here - and hot water for showers/baths!

With pleasure. They will expect feeding afterwards though!

Anya can you send them round to my garden please - ash saplings all over the place
(if not - any hints please? )

DS and DDIL take their DC on Scavenger Hunts:
www.pinterest.co.uk/explore/nature-scavenger-hunts/?lp=true
www.woodlandtrust.org.uk/naturedetectives/

Get her out of doors. There's so much to learn in nature and in towns.

I watched four of my grandchildren 5, 6, 7 and 10) trying to uproot an ash sapling (in my garden) which I'd asked them to pull out. It was very narrow but about 8' tall and very bendy. They rocked and pulled it back and forward in an attempt to uproot it; then they bent it over level with the ground, all stood on it hoping it would snap, but it just sprang back upright. They tried twisting, sawing, digging it out.

It took almost an hour of ingenuity and team effort to eventually remove it by which time it was raining and they were wonderfully mad-splattered.

What they learned about the properties of materials, physical forces and other laws of physicals was beyond any book. And the amazement at the root structure when it was eventually uprooted was pure botany.

Before anything else, encourage her to ask questions!
If you can't answer then show her the 'finding out' process.
Don't get stuck in academic mode - introduce art/craft work which is an excellent way to give one-to-one attention and encourage communication and social skills too.