I have used the Philips I²C chip called PCF8591. It has 4 channel A/D input and one channel D/A output. It fits in 16 pin package. Nice part.....but is 8 bits resolution.

You could look at Cygnal 8051 derivatives. They have processors with A/D and D/A on same part. Some parts are 8 bit and some are 12 bit.

Michael Karas