What teens and the young twenties group likes is convertible mittens. Here's a tutorial that shows you how to convert any mitten pattern into convertible mittens, the flip type kind. It's very easy to do. The only thing different I did with this was to make thumbs with a slit in it so you can pop your thumb in and out for texting and digging out bus change. You make these on the back or inside of the thumb. After picking up stitches for the thumbs, I knit a few rows, then cast off the inside thumb stitches and recast them on in the next row.
As to the type of yarn needed, it depends on your climate. I have two sets of mittens and gloves here in Wisconsin. I use the lighter sport weights for spring/fall when the temps hit 30-40. The worsted and bulky weights are for anything below that. Although, this year the weather has been really strange. We've had 40 some temps during January, something I've never seen in my lifetime. It's usually 60 degrees colder. From what I'm reading online in my weather forum, the whole country is experiencing this strange mild winter. I saw geese flying north last week. Even they're mixed up. You might want to go with the sport weight if your weather is anything like ours.