Hi!
I'm also a veteran of the day-trip-while-on-vacation-in-Europe thing. Lot's of fun!
Even if it's only one day, it'll be a great one, and you won't have to give up much time from your European holdiday.
My main tip, which I'm sure everyone else will echo, is to just visit
Disneyland Park. The Studios are nice, but don't have nearly enough to warrant going to when it's your first trip.
As for choosing when to go...well, the park will be more crowded on the weekend, so good choice avoiding those days. As for Monday vs. Tuesday, see what you want to do in Paris on the day you do not visit Disney. If you want to visit the Louvre on that day, for example, keep in mind that the Louvre is closed on Tuesdays. Or, if you wanted to do nothing but shopping on those days, remember that some small shops (not nearly a majority, but a good number- definitely not the major department stores) are closed for half a day on Monday.
Also, are you using a Paris Museum Pass? They are a great option for visiting the museums, IMO, but they require that you use them on consecutive days, which is another thing to keep in mind.
One more thing: possibly keep your visiting date flexible, just in case it turns out that you hear right before your visit than one day will have terrible weather and the other will be beautiful.
As for the park itself, my main tip is to get there shortly before the official opening time. Each time I've been, whether the park opeened at 9 or 10, things did not start to pick up at all until around 11:30-12, so basically every ride was either walk-on or close to it.
Plan to stay at the park until closing time. I love the park at night, it's so magical!
As suggested, do get tickets for the Lion King show. If they only have tickets left for one of the French shows, still get them. About 90% of the show is sung, and the singing is in English in all of the shows. It's just the 10% (less, actually) that's spoken that alternatives between French and English.
The only rides that I've ever seen get very long lines are Peter Pan and Thunder Mountain. Either ride them early, or get a Fast Pass sometime before the early afternoon (the fast passes get distributed rather quickly sometimes).
If you're a WDW veteran, some of the attractions will be very familiar (for example, Start Tours, Honey I Shurnk the Audience).
My favorites include Phantom Manor (DLP's version of the Haunted Mansion), Thunder Mountain, Pirates of the Caribbean, Peter Pan, It's a Small World (DLP's is better than WDW, IMO), Snow White, Pinocchio, and the Lion King show. Space Mountain is also great, and I think it will be open by then (a new version they're preparing!) But really, everything is great, and even with one day, you can see everything, as long as the park is not terribly crowded that day.
See the parades! I love them!
As for meals, for regular sit-down restaurants I'd recommend Blue Lagoon and/or Auberge de Cendrillon. These are my favorites in the park. For a character dinner, consider Inventions at the Disneyland Hotel (this hotel is literally over the entrance to the park).
HAVE FUN!!!
I may add something later if I forget, but I do hope that this helps!
