You can use parallel ADC too. It will use 8 line for data, 3 line for channel selection ans 2-3 control line still you will get enough pins to interface other thing and Both RTC DS1307 and I2C protocol based EEPROM can be interfaced to same pins and implementation of software based I2C protocol on 8051 controller.
If you want to use serial ADC then there is ADC8031 which uses serial inteeface and MCP3204 from microchip which uses SPI serial interface.