I would probably say dont try and cover everything with one board, SPI and I2C are very different beasts to just shift registers in that they use addressing and registers, and some devices require values to be written to setup registers before they will even respond to anything else,
You could still take your shift register board and challenge yourself by using the shift register to control 5 RGB leds (3 colors and 5 enables), by using direct pin writes it would be more than easy to not only make it appear as if all 5 were on at once, but if you really wanted to push it could possibly get say 4 bits of PWM by loading the shift register with new data fast enough, (5x 4x 60Hz = 1200 updates per second), or 9600 pin toggles, which gives you over 1600 clock cycles per update to play with what you want to do in the next update
As for where to head from here, yes you could defiantly go for a light / colour sensor as i would imagine a large number of the sensors would use i2c or similar to read out the data from them, or grab a cheap 10 DOF (Degrees of freedom) board with accelerometers, gyroscopes etc, as you get to play with the sensors and they also have a ridiculous amount of settings which means you can dig in and figure out how to set it up etc,
as for i2c / spi I/O expanders, i have not touched them myself for no other reason than i have not yet needed them, they have some merits, but short of more advanced peripherals like additional PWM / adc's, i fail to see the need for more than shift registers,