Assuming you're hardcore Disney --- you have 10-16hours to do things depending on the day you're seeking. With adults - easier.
1. Maxpass. Be ready to troll for passes on
MDE with all three of you on it to grab cancellations and releases throughout the day.
2. Make a list of the rides you DON'T get at WDW and if which ones are Must (ALL!) or Nice to (none!--but really do you need Jessie Critter or Inside Out Whirlwind?)
3. Park hopping is EASY.
Remember, the entirety of DLR is smaller than MK alone. You'll probably be most alarmed by the density of people in the common spaces compared to the wide open spaces of WDW. The only time where WDW felt busy for us was the corridor between PeterPan and Small World -- while that's almost normal for most of DLR.
If your day is Magic Morning in CA, do radiator springs and get it out of the way. Mania and Sorin' are duplicates unless Soring California is running - which would make it a Must if so (IMHO). Then aim for an evening fastpass for Radiator springs as the morning progresses and grab those -- very different experience for RS at night and a bit unique. Time that with World of Color showing.
That said, we also really enjoyed the duplicate rides as some of them have variations and we really enjoyed the differences -- for better or worse -- like POC, Big Thunder, PP, small world. -- just depends on what are your priorities for 1 day only
Edit: last thing -- if you really love your Space Mt, -- don't ride DLR -- you'll never want to go back to WDW-MK!