Arrive at STN 3 hours before a flight after the early morning rush ? Better have a very good book to read...
Expressflight - seriously though, the main departure lounge with all the shops is not a pleasant place to hang around for any longer than absolutely necessary. If you have plenty of time, don't want to pay for a lounge, have no trouble walking for 5-10 mins, and want a bit of peace and quiet, make your way to the Ryanair pier (gates 41 to 59 I think) and head towards the end of the pier where there's usually plenty of space. Whatver you do, don't jump onto the transit system in the expectation you can get back to the main terminal area later - you can't.