Weekend in Paris
Or in Venice, Rome, Barcelona, London, New York, Cairo, Bangladesh... The list of great cities everyone should visit at least once in their life is immense. We mention Paris in the title because it's the City of Love par excellence, but if you're planning a surprise weekend trip, any of the aforementioned cities will be perfect.
Prepare the surprise thoroughly, well in advance. Tell your partner that on whatever weekend, even if it's a month away, you're invited to attend a wedding or that some friends of yours, who are in on the plan, have organized a party at their country house... any excuse that prevents them from making plans for the travel days will do. Also, make sure they don't have any commitments during those days. To do this, talk to their work or class colleagues to ensure that their absence can be covered without issues. Once this is done, get tickets to somewhere glamorous and get ready to give one of the biggest surprises you can give someone.
Imagine the scene: your partner arriving with their suitcase for the weekend, and you're waiting for them in a taxi whose destination is the airport. Don't let them know the final destination until they're at the airport ;) Lastly, don't neglect their luggage: ask someone in their family to help you get suitable clothing for the city you're going to.