Replace your vanity with a pedestal sink or with a new wood model that has a stone counter. Add a new mirror and faucet. Or keep your current vanity, but replace your toilet and faucet and add a new vinyl floor.
Improve lighting and ventilation with a new combination light and exhaust fan. One with a heat setting might cost a bit more but will keep you from getting chilled when you get out of the shower.
Add a set of sconces on either side of the mirror or medicine cabinet.
Update towel bars, hooks, toothbrush and toilet paper holders, and cabinet hardware. Add matching toiletry and towel shelves.
Switch your standard showerhead to one with multiple settings, including a pulsating or massage setting. See the showerhead report (available to subscribers), for models that produced the most oohs and ahhs in our tests, including one that helps save water.
Keep your towels toasty with a heated towel bar; some cost $100 or less.
And make sure you rent a Waterloo Washroom so you're family isn't fighting over 1 bathroom. :)