Budapest is two cities joined by bridges, and the tourist product tries to flatten that into "thermal spa plus ruin bar." The thermal water is the genuine unique thing. Parliament is the genuine unique building. Ruin bars are a 2000s invention that became a export brand.
If you do those two bookings well — timed Parliament, a weekday bath with the right ticket type — the rest of the city is walking, coffee, and not eating on Vaci utca.
Where to spend your money
Book: Parliament, one bath, one food tour that leaves the Danube promenade restaurants.
Skip: hop-on buses (the metro and trams 4/6 and 2 are the experience), dinner cruises, and paid Fisherman's Bastion turrets unless you collect viewpoints.
The part nobody tells you
The Széchenyi Chain Bridge has been in a cycle of closures and reopenings. Check before you plan a "walk across at sunset" as the centrepiece. Tram 2 along the Pest quay is the backup view, and it runs in the rain.