Trek 7500 Hybrid:
If a computer is considered an upgrade, that's what I did first!
Then came bar ends so I could alter hand positions, mirror. That's the way things sat for a couple of years, though went through a couple of mirrors and different computers.
This year I put on clipless pedals w/appropriate shoes. Big difference! Upgrade to a Garmin 305, so I'm probably "maxed out" in that department.
Next year it's a whole new bike! Starting to look around for a road cycle now . . .
Constant effort to upgrade the rider! That's not so easy any more . . .