Are they completely new to fly fishing or just green? If they have never done before, teach them to cast on a lawn and make sure they know the blood knot and improved clinch knot. This will save you and them a lot of frustration when you get to the river, It'll be a lot more enjoyable for you both. That said, good on you for sharing your addiction, I'm sure it will soon consume their lives too. South fork Sno would be a good choice. Have fun!