Buy some big, thick, fluffy towels, even if you don't use them they will look lovely.
Buy some matching candle sets for the side of the bath or for on top of the lav.
Try to avoid those little knitted dolls for your loo roll however (ha, ha, ha).
Simple white tiles are cheap but look gorgeous teamed with warm natural colours on the walls. then simply add fresh fluffy towels and bath mats. The only thing I would recommend slurging on is a gorgeous mirror as it instantly lightens the room and makes an attractive focal point!
Candles every time;) Long thin ones, short fat ones, scented and unscented. You cant go wrong. Another tip is if you find a particularly prettily decorated candle just light it till the wick goes down a bit and you can insert a tea light in it. The tea light wont be visible and the candle will have alonger life;)
Buy some nice inexpensive pictures that you can hang on the wall and accent the other colors in the room or if you have a little artist in yourself paint your own pictures to match! ;)
dont clutter. minimalist, makes it look bigger, have big fluffy towels and good quality nicely bottled shampoos etc. and make sure it smells nice with good lighting.