The first adapter I'd suggest would be for M42 mount. With this, you can use a myriad of vintage lenses like the Pentax Takumars, Zeiss Jena, Pentacon, Helios, Jupiter etc.
Olympus OM, Contax C/Y or Leica R mount also have their merits, but lens prices go up quickly there.
I really like the Takumar, the Contax Planar and the Summilux-R, which all have totally different characters. Also the Helios 58mm is a special one, it's quite a nice character when used wide open. With the 50mm vintage it is difficult to go wrong, you must only be careful not to pay too much.