We went in June this year for my 50th and loved it (also went for three nights)
We stayed at the Hotel Continental which is in the Jewish Quarter so handy for Szimpla Kert and lots of other ruin bars but a decent walk (or 5-10 minutes bus ride) into the main part of town.
Dohany Street Synagogue is the largest in Europe and definitely worth a visit and the one hour tour was thought provoking for somebody who knew nothing of the history of Budapest.
szecheny baths (Gellert baths were partially closed for renovation when we were there), The Market, Margaret Island (great open air pizza place there), Fisherman’s Bastion and the castle (take the funicular). Loads to do.