Front and rear sway bars, underbody chasis bracing, rear strut brace, Supra 4 pot brake and rotar upgrade, theres still plenty to do!
Cusco sways were probably the most noticeable handling mod I've done, having a nice set of bucket sets compliment it aswell. If you do get bucket seats get nice ones like Recaros, Bride etc as cheap ones tend to get uncomfortable fast.
*edit* if you take it on the track, theres oil coolers (with thermostat) and powersteering coolers which are good to do.
Also an aftermarket ECU should give you some gains with the headers/exhaust mods... and if you're doing that you might aswell go quad throttle bodies and trumpets! (Just because I want to see someone do that :p)