My girlfriend and I are visiting Paris for 3 days and Disneyland Paris for 1 day (Day 1, 2 and 4 Paris; Day 3 Disneyland).
What tickets/pass would be the best and possibly the cheapest to buy?
If you arrive before Thursday midnight, buy a Navigo Decouverte week pass. This covers you for everything for your entire trip. If you arrive Friday-Sunday, buy Navigo Easy loaded with two 1-2 zone day passes (Navigo Jour) for your first two days in Paris (you can buy up to 2 day passes on card at the same time). Then use single-use RER tickets for Disney and back to Paris (you can't load these onto your Navigo Easy, use the paper coupon tickets). And finally another 1-2 zone Navigo Day Pass on your Navigo Easy for day 4 in Paris.