Depends on your budget. In order of cost:
1. Buy a couple of tester pots and paint a few geometric shapes on stretches of wall.
2. Paint one wall a bright colour.
3. Paint the tiles.
4. Put up a big colourful noticeboard.
5. Swap to brightly coloured kettle, toaster, visible mugs, flower pots in the window etc
6. Paint the whole kitchen.
7. Change the light fittings.
8. New worktops.
9. Change the cupboard doors - apparently they can look "so old fashioned" according to one kitchen firm I spoke to.
10. New bright room doors.
Depending on your boiler you may even find just cleaning the walls and ceiling thoroughly makes it look better.
Have fun.