My niece asked me this question in February of this year (& i forgot to blog about it). My niece is 11 (she was 10 when she asked).
This was a tricky question to answer...of all the possibilities i could have said, like cures for diseases or ways to end poverty, I decided that I would invent a door.
This wasn't any old household door, it was a special door - a portal to anywhere in the world.
Before you stepped in you would key in a postcode of any place you needed (or wanted) to be: a holiday destination, a place of work, a family home (on the other side of the world), a friends house.
Then when the door had a fix on the postcode it would be activated.
All you need to do to use it is open the door, step through (with your family/suitcases etc) and when you close the door behind you, you are teleported to that destination.
If there wasn't a door at the destination then you would be able to get there - but you'd know before you set off.
The door would reduce the amount of other forms of travel (rail, sea, air, car) and connect you to anywhere in the world, saving you time, money and the inconvenience of delays/traffic jams and all that stress. For those people who suffered with travel sickness or fears, then this would literally open up a whole new world for them. You could live absolutely anywhere in the world and use the door to commute to work, no more delays due to low rail adhesion, no way!
I would use it to visit family and friends, knowing that the journey wasn't going to take an age, or be disrupted by weather, timings, cost or vehicle issues.
So my weekend might look like this:
Friday evening I would visit my mum after work, saving myself a 50min train or 1hr car journey and we could catch up, staying for a couple of hours before going home for tea.
I'd spend a leisurely Saturday morning at Made By Bob in Cirencester with my friend Kate, and Saturday afternoon in Made Cafe Whitley Bay, doing something crafty. And Saturday evening Mr Lisa and I would pop over for a bite to eat with my friend Auntie Gemma & her family in Beijing.
On Sunday we'd have breakfast in our own bed :)
Then for the afternoon we'd arrange to meet up somewhere child-friendly and cycle friendly with friends (possibly somewhere Scandinavian) and have a blast on the bikes followed by cake.
And Sunday evening we'd meet up somewhere in Newcastle for dinner with friends, staying out as late as we wanted - we'd go through the door to get home.
The doors would exist in every town/village, they would be free to use and always available. No one would own them, and no one would get rich from them. They couldn't be broken or vandalised and would be there for everyone to enjoy.
This might reduce the number of journeys on our travel networks but it offers so many amazing possibilities.