Once you visit Budapest you will want to visit this city again and again. The city is split into 2 parts Buda and Pest and you can easily spend and entire week there.
Sightseeing – At Pest I will recommend to visit parliament building, to take yellow metro line to Hosok tere ( Heros square , Zoo and Szesany Spa center are few minutes walk away )
Buda side has a lot of history and museums . You can spend one day at Buda Hills, visiting Fisherman bastion and Matias Church.
Other attractions in the city: Margit islande located on danube river , Vaci Utca famous walking street of the city with many shops and restaurants.
Restaurants : I will recommend Lugash –located in the city centre opposite Basilica , the place with best Goulash for sure.
For meat lovers there is a paradise place called Sir Lancelot (2 min away from Nyugati metro station ), except great atmosphere and design you really get nice piece of beef for reasonable rate.
Shopping ( famous female hobby ) - don’t waste your time and money on Vaci Utca, take a metro to Nyugati station and spend your day at Westend shopping centre. If its still not enough take another 4 metro stops and visit Duna Plaza – biggest mall of the city ( credit card companies will be proud of you )
Hotels – I will recommend Hilton WestEnd, as of location, shopping, atmosphere and of course SERVICE, something that they are specialized in.