Either a mural poster or you can create a collage with your favourite vision or pics.

You could paint the wall with magnetic paint. Then you can put up many kinds of postcards, posters, and so on.

Thank you for the ideas. Today I found a perfectly good hammock in red and white string in an op shop, so thought I'd have a go at draping the stuff across the wall and using wooden pegs to hold postcards and drawings on.
By the way, does magnetic paint really work?

I used Rustoleum's magnetic paint in a mural I painted in my house a couple years ago and it works OK. Not as strong as a fridge, but holds up single sheets of paper. Helps if magnets have good strength as well.
Magnetic paint does NOT WORK well over orange peel or walls that have texture- they should be super smooth. Three thin coats, even 4. It is a black primer, and you probably don't want to put to many thick coats of paint over it either.
With all those caveats, people always comment on our magnetic wall, and it is super cool to have a great idea or dream and to magnet it right up, move stuff around, etc.

Here is a photo of our "dream tree" magnetic wall.

OK, since creative spaces are kind of my thing, I will add two more ideas to this comment thread! I painted a door in my studio with a simple mural inspired by a map, and then covered in with dry erase paint from http://www.ideapaint.com. I have tried other dry erase paints in the past, and not been pleased with the results. They stain easily, don't erase so well. Idea paint is expensive, but it works better than my Target white dry erase board. I really love having a door to write notes to myself on.