In itinerary order. Where official booking is the better route, the official link comes first and no reseller product is offered.
Evening Danube cruise
Day 3
The single evening anchor of Day 3: ordinary sightseeing, not a party or a multi-course dinner. The operator is Legenda City Cruises, and the listing and the operator's directions identify Dock 7, Jane Haining rakpart; follow your booking confirmation for the actual departure. Choose a departure that matches sunset on your date, since "evening" does not guarantee darkness in summer, and compare covered seating and cancellation conditions.
Széchenyi Thermal Bath
Day 4
Day 4's long afternoon block, in place of an exhibition rather than alongside several interiors. Compare the date, entrance, locker or cabin provision and cancellation conditions with booking directly; direct booking is a good choice when its terms and ticket fit you better. Check the current admission rules: at the time of writing, children under 14 are not admitted. Bring a towel and slippers, and check any pool-specific cap requirements.
Parliament
Day 3
Day 3's morning interior, if you want it; the exterior is a free alternative. Official booking is the primary route, so check tickets, prices and release dates on the official page. Avoid "Parliament tour" resellers whose inclusions, entry status and language are unclear.
Basilica and Matthias Church
Days 1 and 2
Prefer official tickets. The Basilica's optional panorama fits Day 1, and its interior's Sunday sightseeing window starts later. The Matthias Church interior fits Day 2, and the church and the tower are separate choices. A tower or terrace is optional, not another mandatory admission.
Opera and park museums (optional)
Optional
These replace another substantial activity and are not additional essential paid attractions. The Opera auditorium may be inaccessible during rehearsals, so do not count on it on every tour. Book directly with the institution.