One of the most annoying noises the MR2 ever makes is the seatbelt warning buzzer that sounds when you start the engine without first locking the drivers seatbelt. It's also one of the easiest mods to do. Definitely one of the most relaxing! All you need to do is disconnect the relay buzzer located in the firewall, just behind the driver seat at about shoulder level when seated - almost immediately below the point where the drivers seatbelt comes from the b-pillar, behind the drivers seat.
Remove the two screws, ease out the panel, and you'll see the relay. Simply disconnect the plug and you're done. No side-effects at all.