J.,
I'm a third time mom - so I have 6, 4 and newborn. We have consciously made the decision to limit our activities as of the latter part of my pregnancy (this Fall) and now (Fall and Winter) due in part to keeping everyone a little healthier.
Now I am not couped up - I go to the grocery store, schools, drs appts, etc. as needed. But we decided to limit the extra-cirricular activities for now. We have explained it to our other kids that Mom and baby are tired (being up at nights) and he needs his naps. Additionally, we don't want to get him sick since he can't fight germs like the big girls can.
Although I too am exclusively breastfeeding, that doesn't mean he can't get sick. The drs and nurses at the hospital insist we are being smart. And our pediatrician agrees that there's nothing too important to have to take him to unnecessarily.
The reality is that it's not just the people touching or sneezing on the baby - it's the air. The germs are in air. It's not worth taking the baby out if you don't have to. Even though you're nursing and babycarring he simply doesn't have the same immunities you do - he can STILL get VERY sick, just from the common cold.
As far as your 19 month old, I don't think that it would be a HUGE disruption to her life if you do other things - like borrow movies and library books, and do at home crafts. I think as long as she isn't forgotten it doesn't matter what you do with her. Besides, your playgroup friends may be willing to pick her up to come out to play as opposed to having YOU bring the little baby out.
So my advice is skip the outings and playdates at least for awhile. It's not worth getting your new baby sick to go to a playdate. You won't forgive yourself.
Be happy and healthy!
Sara