I've mentioned some of this stuff before, so bear with me if this sounds a bit repetitive ...
Star Wars
Made the laserfire from Imperial blasters green.
Given characters unaffiliated with the Empire or Alliance their own distinctive blasterfire colours (yellow for good guys like Han, blue for bad guys like Boba Fett).
Kept Han and Luke in their stormtrooper armour until after they got off the Death Star.
Like has been mentioned by others here, I'd have made the duel between Ben and Vader more elaborate.
The Empire Strikes Back
Made Luke's lightsaber cyan instead of azure to distinguish it from Ben's lightsaber.
Played up the whole "Han-Luke-Leia" love triangle by showing more love scenes between Luke & Leia.
Made the laserfire from the probe droid and walkers green.
Shown more AT-STs on Hoth.
Hired a beautiful young actress to play the body of Palpatine (I would have kept Clive Revill to play the voice, though).
Return of the Jedi
Replaced the Death Star II with an Eclipse-class Star Destroyer, and have it serve more as a setting rather than as a MacGuffin.
As mentioned for TESB, I would have hired a beautiful young actress to play Palpatine and brought Clive Revill back to play his voice.
Introduced the existence of other Sith Lords/Dark Jedi beyond Vader who serve the Emperor.
Placed Jabba's headquarters on a new world instead of on Tatooine.
Changed how the whole "rescue Han from Jabba" sequence goes to make it work better.
Have Han show some distrust/resentment towards Lando for what happened between them on Bespin. Lando would only regain his trust later on when he chooses to fight in the Battle of Endor.
Removed the whole "Leia is Luke's sister" bit.
Removed the whole sugar-coated "from a certain point of view" crap. Ben would admit that he lied, but that he did it in Luke's best interests.
Exchanged the Ewoks for a more intimidating race (I prefer Trandoshans, personally).
Exchanged Palpatine and Vader's "if you kill me, you'll join me" method of converting Luke to the dark side for something that makes more reasonable sense.
And, finally, either have Han die so that Leia can finally be with Luke OR have Luke choose to let Leia go so that she can be with Han.