Avalon Streaming simple example

It depends on what you need but usually that triplet of signals is all that's necessary. There are other signals in the Avalon-ST spec and depending on your needs you might need them:

error: If you have to transport error information in the data stream

channel: If you have to transport a channel number in the data stream (i.e. interleaved channels in the same physical stream of data)

sop/eop: If you have to denote the start and end of a packet in the stream (for example if you were transporting Ethernet packets this would be handy)

empty: If you need to denote unused symbols in the data stream. (for example if you were transporting 32-bit data of Ethernet packets you can use this to mark how many symbols are empty at the end of a packet if the packet isn't a multiple of four 8-bit symbols).

Technically you can have an Avalon-ST port with just the data port but in general streams usually don't transfer data every clock cycle or have to backpressure so that's why you typically include ready and valid. The Avalon-ST protocol is very flexible just like Avalon-MM so there are very few mandatory signals to include.