I'd recommend something like this
3-tier steamer set. They're very versatile, giving you an extra largish saucepan, and you can boil rice / pasta / potatoes in the bottom whilst steaming your veg in the top. Also if you have veg that would normally cook at different rates you can stack the faster cooking veg in top and it will get slightly less heat (as some of the steam will have condensed on the lower basket). They're pretty ubiquitous, so shop around before you buy, and don't worry about getting a larger one than you think you need - remember you can always cook less food in a large pan, but you can never cook more food in a small pan!