One of the best places to be in Budapest is at the Szechényi Termal Bath. This is probably the most famous spa in Budapest, and you can combine inside and outside pools with different treatments. For example the price of a 30 minute massage is 3000 Forints, or approximately 12 Euro.
The other famous bath/spa is the Gellert Bath, in our eyes not as good as the Szechenyi, but it is still worth a visit.
What is also good is that the Szechenyi Bath recently added new lockers to the entire complex, which makes things easier, nicer and improves the entire feeling a lot.
The Szechenyi Thermal Bath
The Szechenyi Thermal Bath