Using ATTiny84 digital output pins
TL;DR Nothing special needs doing.



All IO pins with the exception of the Reset signal can be general purpose digital IO.

The reset signal can be used as digital input, however in order to do so you have to disable ISP in the fuses meaning only high-voltage programming can be used to recover the chip. I would advise against using it if you can avoid it. 



The ISP pins PA4, PA5, and PA6 can be used as GPIO without disabling programming, hence the name In-Circuit Programming. A little bit of care is needed to make sure that your circuit doesn't interfere with programing capabilities.

If you use the ISP pins for things like push buttons or LEDs, you don't need any extra circuitry as these won't affect the programmer.

The simplest solution for connecting to external hardware to this is to just add jumpers (either solder or pin header) between the rest of the circuit and the MCU - you can then disconnect your circuit to prevent any interference with the programmer.