I treated myself last week to a 2nd hand prime lens. Cosinon 35mm f2.8 - M42 fit, with adaptor for the Sony, fully manual. I already have one but wanted to upgrade to a better one & promised the other one to Crowthius - [link]
It's a tiny little lens but the image quality seems really good. Reminds me of my Minolta 50mm Prime. If you've not used a fully manual lens it's really good practice. When i bought my first SLR camera in 1982 you didn't get autofocus anyway.
35mm focal length sees things as your eyes do, same field of view, no distortion of perspective. You get what you see !
I only paid £14.25 for the lens including P+P from Ebay & the chipped adaptor was £13 from Ebay. The chip means you can use focus confirm on the camera. It tells the camera a lens focal length & maximum aperture so you can use Aperture priority. You just set the aperture manually & the camera selects a apropriate shutter speed. You can also use it in manual mode.
The lens is no good if you're in a hurry, ie no autofocus or autoexposure but for stills, portraits & landscapes it's great.

Also i bought another lens brand new. A Falcon 8mm fisheye f3.5
This is a Minolta/Sony bayonet mount & gives you a 180 degree diagonal field of view on APS-C ! No vignetting. I've lots of shots envisaged for this lens but it only came today & i probably won't get out with it until the weekend (weather permitting) This lens is also fully manual but no chip to tell the camera what size it is & no focus confirm..
Unlike the 35mm prime this is the complete opposite & will give extreme perspective distortion. Lots of curved lines that should be straight. Watch out for extreme architecture shots in the coming weeks
I can't wait to get going - i'm bursting with ideas & enthusiasm.
Happy Shooting & chat soon