The New York Times suggests hiring a private driver for half-day weekend tours, which will cost around $234. On weekdays, you'll find them for about 40% cheaper than that.
Porto, Portugal, which is north of Lisbon, is comprised of 18th- and 19th-century buildings as well as newer trendy shops and restaurants.
Airline search sites like Kayak have also noted the growing popularity of Portugal as a tourist destination. Kayak saw a 46% increase in searches for flights to Lisbon compared to this time last fall.