That's the same problem as Raspberry Pi. For output-only digital signals there are some 5V peripherals that will accept 3V3 input levels. And for input-only, yo

That's the same problem as Raspberry Pi. For output-only digital signals there are some 5V peripherals that will accept 3V3 input levels. And for input-only, you can use a potential divider. It's bidirectional where you need a proper level shifter. But you can get really small cheap ones. I've used them a lot on I2C interfaces. If you're careful you can design the entire system around I2C and then you only need a single level shifter. But I've not built many things around ESP8266 just yet. Still learning about it. It's an incredibly cheap and powerful platform though.