Using Navigo Card for (mis)Adventures on Weekends

Someone may correct me, but I think that Saint-Denis would be past zone 3, the Basilica there is a must see if you'll be in Paris for a month.

St. Germain-en-Laye and Chateau de Malmaison are two more.

Saint-Denis is in Zone 3. See http://www.ratp.fr/informer/picts/plans/pdf/reseaux/zones.pdf. Well actually some parts of Saint-Denis are in Zone 2 but as you note, it will not matter on weekends. What about a weekend trip to Provins which is a bus ride (Line 47, I think) from the end of RER line A?

I visited the lovely medieval heritage merchant town of Provins on just such a monthly pass last year. There are several marked walks going through old and new areas of the town as well as several demonstration shows.
I also went to the town of Auvers sur Oise, Van Gogh's final home. It was great to follow maps leading to the places where he painted some of his most famous pieces.

It doesn't matter what zone St. Denis Basilica is in (it is I believe in 3) because a zone 1-2 ticket will take you anywhere the metro goes and there is a metro stop at St. Denis Basilica (named just that). So you can go there any time. The metro occasionally laps over into outer zones e.g. St. Denis and La Defense being obvious examples. It only requires the basic t+ ticket or 1-2 charge on the ND.

I would not load the ND for 3 zones -- there is little of interest in zone 3 -- just get the 1-2 and then use the dezonage on weekends to go anywhere in the 5 zones which is a privilege of a monthly pass.

Some other great spots to visit: St. Germaine en Laye, Auvers sur Oise, Moret sur Loing, Cressy la Chapelle, Sceaux, Ecouen, Rambouillet, Malmaison, Chantilly & Senlis. You can see some of these from our trips here:
https://janettravels.wordpress.com/category/day-trips-from-paris/

Thanks @janettravels44, but our flat is in Colombes, so we have to get the Zone 3 pass anyway.
And thanks everyone - I will surely investigate these options!

Ah yes, I should have guessed you had a reason for a 3 zone pass and commuting from zone 3 is the perfect one.

One month, zone 1-3 that you need anyway, is 89,20 euro. Zone 1-5 is 116,50. If you can only travel to zone 5 during weekends because you're working or attending classes during the week, then that makes sense to save that 27 euro per person. But I think I'd rather pay a little extra and then have that freedom to go whenever I wanted and not wait for Saturday or Sunday.

And adding on the ticket to and from CDG (zone 5) that's another 20 euro, so the difference now becomes 7 euro.

If I remember correctly, zones apply only to the RER and TER trains. You can go anywhere in the Ile de France on the Metro with a single ticket. Your Navigo pass, even if is good only for zones 1-2, will take you to the farthest reaches of the Metro in the Ile. You need zones 1-3 to go from inner Paris to La Defense on the RER, but not on the Metro. There is no ticket check when exiting the Metro as there is on the RER and TER. Also, from July 15 to August 15 this year there is no zonation on the RER. Thus, you can go anywhere in the Ile de France on a Navigo zone 1-2 pass. I'm not sure if this applies to the TER.

Thanks @bobrad20. I would certainly take advantage of this, except our apartment is around a 10-min walk to the Colombes station, but it would take about a 45-min walk to get to the nearest metro! For only an extra €19 for the month, I will save that extra 35 minutes and use the energy towards eating macaroons :) but I do remember reading that somewhere and I am glad you reminded me. We arrive 3 days before the start of a new month so it may come in handy just buying a zone 1 book of tickets to share and stick with the metros until we can get our Navigo card.