Prague's elegant cafes do not disappoint -- try them all, or at least step inside and marvel at the splendid interiors.   But after this obligatory sampling, you may find that Cafe Slavia is one worth going back to again and again.  Neatly perched along a main thoroughfare near the Vltava River, it's an ideal spot for people watching (both inside and out).  Slavia serves substantial hot meals and treats, plus a good selection of coffee and tea (hot or iced), and a full bar serving cocktails, beer, wine, and aperitifs.