What do you recommend. Two adults, arriving at CDG terminal 2 on April 30 at 11AM. First to Brussels Midi for 3 days then back to Paris CDG. Then from CDG to Paris Luxembourg station 5 day stay, then back to CDG. Which tickets do you recommend. Thank you

Assuming you're immediately / directly taking the TGV from CDG to Brussels Midi on Tuesday, then back to Paris on Thursday or Friday, you should still purchase the Navigo Decouverte week pass for one week upon arrival on Tuesday, even though you won't use it till Thur/Fri when you're back at CDG train station to get to Paris.  If you return to CDG on Friday you cannot purchase Navigo for use through to Sunday evening, leaving you only with more expensive options.
On the Monday (or anytime from Friday onwards) add another week to your Navigo card to use it for your remaining days in Paris and for the trip back to CDG airport.  It's still the cheapest and most convenient option.