As said above .. the options open to you depend on what types of input and output you want to use, where the solution may 'live' may also be important due to extreme conditions? and your budget could also steer the direction you take..
for example in my home automation experiment i take input from the cheapest battery powered sensors that provide temperature, humidity, movement (pir) and door/window status wirelessly and control a/c mains smart sockets also via wifi to turn appliances and lights on/off However my choice to go wireless with sensor and output control carries a cost premium, as a bluetooth temp/humidity sensor (aliexpress etc) costs circa £10 while a number of sensors suitable for direct connection to the pi gpio could probably be sourced for £1. And a wifi smart socket (retail version) will also cost at least a tenner probably £15 against a hard wired relay option that could probably be solved for about 10%.of that cost.
If your worried about giving away hints about your secret project destined to destroy the world or make your fortune, focus on 1 x input or output to start with..