For an even larger screen and Lollipop, the $50 Amazon Fire 7" tablet is a good deal. I got two for $35 on the Black Friday sale. It is actually a pretty decent cheap tablet. Has cameras, WiFi, Bluetooth and USB OTG. Missing GPS and a light sensor.
Look on the XDA forums for root, mods and custom ROMs
http://forum.xda-developers.com/amazon-fire. I have both of mine flashed with CM 12. The stock Fire OS is also based on Lollipop, but it is highly Amazonized. CM 12 is much nicer. Someone did a teardown over on XDA.
I don't have any particular project in mind right now, but I figured for the price, I could dedicate the tablets to some task. Maybe interface to a microcontroller or something. If I blow them up, I won't feel too bad about it.