Warsaw: The Polish capital and Political as well as financial heart of Poland. The most destroyed city in the entire world during WW2 (~90% of all buildings and ~99% of all monuments and historical buildings) but largely rebuild afterwards, especially in the past 25 years.
The most important part of your visit to this city is the Warsaw Uprising Museum. You need to visit this to understand the city, It's people and the whole country in general. The uprising is still an open wound in the hearts of Poles and symbolises the loyalty of Poles towards their country and fellow Poles. https://www.youtube.com/watch?v=BGb6f0xw3f8 Be sure to also buy the ticket for the short movie 'Miasto Ruin' when buying your ticket for the museum entrance. It's really worth the few extra PLN and try to maybe make a donation at the entrance or buy something from the souvenir shop, It's what keeps the museum open ;). Also try to make this one of your first things to visit. You'll be more impressed by how the city has transformed and how close it came to the point of utter destruction.
Visit the Stare Miasto (Old Town), Royal Palace and the krakowskie przedmieście Although completely destroyed after the uprising it was later rebuilt using paintings, old photo's and other pictures and documents to replicate it a 100%. They did a extremely good job and the buildings really look original and not just 70 years old.
Visit the Palace of Culture and Science. A 'gift' of Stalin from the Soviet people the the Polish people. It was made entirely by Soviet workers (of which 16 died during the process) after the war and was the tallest building in Poland for the next decades, and is still the highest clocktower in Europe. After the fall of communism there was fierce discussion about the fate of the building. It was seen as a symbol of Soviet oppression but also had a significant historical value. It was eventually not demolished and other skyscrapers were built near it so it wouldn't be as prominent anymore. People still refer to it as 'Stalin's Penis' and is still subject to discussion. When you enter the main entrance you can buy a ticket with which you can take the lift to the panorama view point which will give you an excellent view of the city and its surroundings.
Visit the Army Museum if you're interested in military history. It has some really nice pieces, even a capsule that was used in a Spoetnik space mission.
Visit the 'Zloty Tarase' shopping mall. It's beautiful and has plenty of shops your girl and has plenty of options to lunch.
Visit the Łazienki Park with the famous Chopin monument.
Eat at a 'Zapiecek' restaurant to get some of the best pierogy you will ever get. The have restaurants in the old town and are recognisable by the leaflets they give out and the traditional dresses worn by the girls that work there.
Go to a 'Bar Mleczny'. This can apply to every city you'll visit. These 'milk bars' where subsidised restaurants that were meant for the poor during the Socialist times. Now they offer good traditional food for just a fraction of the price you pay at a restaurant. You have to buy your food at the cashier after which you'll likely be given a number after which you just wait for your number to show up or be announced and you can get your food. It's a really good way to get a** good warm lunch for no money!**