I put a shelf on my kitchen window this week. The 36″ shelf I found fit right between the side jams with just a cm or two shaved off the ends. I put 5 screws down through the top into the sill.
Underneath I added a couple of support brackets, knowing that the cats will insist on sitting on the edge of the shelf. I needed to whittle the trim a bit too to get them to fit.
And the finishing touch was splurging on some lovely little pots from Steins. I put them together using the two pot method I posted earlier for decorative containers.
Since my kitchen is done in yellow tile, the bright cheery colors fit in perfectly.