If you go to Hue don't just trek around the citadel, rent some bikes and explore the lanes and lakes to the north of the citadel, they're beautiful and very few tourists get up there, so there's less hassle.
Also, make sure you go to the beach. It's a 6 dollar cab ride. There's no tourists...just rows of beach huts and wooden Vietnamese fishing boats.... and the sand is golden.
Make sure you eat at the Vuon y Thao, also north of the citadel at 3 Thach Han. You can bike there. it's an old colonial house but for $8 a head you get a wonderful six course meal. Easily the best we had in Vietnam and try the Vietnamese chilled red, it ain't bad. If you cycle don't worry if it feels like you're going away from town and it gets a bit leafy..if you're on the right road then you'll find it as the staff stand outside to greet you.
Oh, and make sure you go down to the river at night for Huda beer but watch out for the big water rats!