I voted for Restaurants as I think this is the area that is least well served with standardised offerings that offers a viable market for a small web design business to take on.
Many of the smaller private restaurants (outside of London at least) are only just becoming aware of the need to promote their business on the web. You would need to research the market of course. It may be promotion of special offers to fill tables during weekday lunch times or evening is critical, pre-booking of parties, etc. Some may not want to offer booking but promote the quality of their food.
Those that have had a first go (done by a friend/kid in the family) will perhaps be ready for a better proposition.
Good luck whatever you decide.
Stuart