A beautiful city! My wife and I visited here in December 2024, so outside of "peak season" for Malta. (Which is during the Summer, for the beaches.)
The first activity my wife and I did was take a popular walking tour of Valletta. I'd recommend this as a fantastic way to introduce yourself to the city and its history, and to get yourself acquainted with what important sites exist within the city, how they are connected, and why they are important.
Other reviewers here have covered what great things there are to see. I second all of their recommendations, from the overwhelming Co-Cathedral, to the stunning Barrakka Gardens, to the grim Underground Valletta tour opened in 2021. All of these can begin to get pricey (especially the co-cathedral!) but Malta offers a 🎫"Multipass" that I would recommend as a way to make the most of your time on the island. As of 2024 it gives access to five sites in Valletta, plus twenty others across the country.
Valletta is absolutely worth your time. Also be aware that during the Summer the city with a population of roughly 7,000 residents receives multiple daily cruise ships often each containing over 9,000 tourists! Lines may be longer and tickets for activities should be booked in advance!