Dear... (your friend's name),
I'm happy to hear that you want to visit my country this summer. I have some advice and suggestions for you to do for three weeks. First you really should wear T-shirts, shorts and summer clothes like that because it's hot in our place between May and July. You also need sun screen and hats. For places to visit, I suggest visiting old temples and pagodas. You can visit some traditional villages like Vong Village where you can try com, a tasty Vietnamese traditional food.
Ok, now let's talk about where to stay. In Ha Noi or other modern cities, the hotels here are a bit expensive. But if you have a lot of dollars in your pocket, it will be enough to stay for a week or maybe more.
Looking forward to seeing you this summer.
Cheers,
...(Your name)