DL just completely revamped Splash Mountain, redid all of the drops, cleaned up the animatronics and the lighting - and the seating in the flume is different from WDW. You'll want to do Space Mountain - it's very different from the WDW version and will have the Hyperspace Mountain overlay while you're there. Big Thunder Mountain is different here, also recently refurbed. I'd also recommend it's a small world, as it's in the big outdoor castle, much cooler than the WDW version, and depending on your dates Pirates of the Caribbean may or may not be open - we've got a 6-week down (starting last week).
Duplicates to skip - Toy Story Mania is exactly the same as WDW, Soarin' is the same, Haunted Mansion was designed for WDW simultaneously with DL so while it's slightly different in exterior appearance, there aren't major differences to the attraction and there isn't a historical component of DL's being original. Star Tours is the same. Tiki Room and Jungle Cruise here aren't anything special. Don't waste your time in Tarzan's Treehouse. And although we did just beautifully refurb Dumbo, I wouldn't say that any of the spinning rides are a must-do at DL, they're the same concept as the WDW counterpart.