You'll need to have something that acts as a buffer between the solar cell (low current and variable dependant on the weather) and the RPi (High (ish) current and needing a constant and stable voltage)
Using 4 rechargable cells should give you a nice bit of headroom, AA cells will do (someone else spotted that this would also give a nice charge rate (C/10 is what most cells like)) but will not give you that long a run time- 2 hours from 1400mAh cells. This is assuming that your RPi will be eating the full 700mA, which it may not be doing if you're just running a keyboard.
You could also use bigger cells (D cell equivalent) which would take longer to charge but run it for longer
You'd also need some sort of power/ voltage regulation and conditioning to get your 6V solar-charged battery pack to give you 5V for your RPi.