I've never driven solo with DD for more than a short hop - on long journeys DH always drives and I sit in the back with her and play games, sing songs, read stories, pick up her toys when she drops them, etc.
This weekend we are going to Wales to stay with friends, and DH isn't coming as he's going to a wedding elsewhere. So I'll be driving, and DD will have to entertain herself to a large degree - obviously I can talk/sing to her, but can't interact physically as I'll be bombing down the motorway (at 70 mph or under, obviously). Like most toddlers, DD has a 20-second attention span, and when she's bored, she howls. Any suggestions for things for her/me to do to try and keep the mood sweet would be much appreciated!! She'll probably nap for an hour or so, but 3 hours is a lot of time to fill...