L.N.
you'll probably be using plenty of taxis (cheaper than here). email hotel and ask about their shuttle. most hotels have airport shuttles so you shouldn't have to take the train. downtowns are huge, so there isn't like we think of it as in the states they're big so you might want to do some research on what you want to see. a 16 month will be bored out of her skull, so nail down parks first, or at least include a park trip at least once a day. i prefer zurich to vienna, for some reason vienna seems very sterile to me. zurich was fun, they almost always have some kind of street festivals (this is pre-bebe era for me and my husband), but we used to stroll around, sit at caffes, then go to street markets, a little sightseeing etc. instead of doing vienna and zurich (which both are german type of cities), you may take the eurotrain to italy, northern italy, might be a change of scenery and architecture for you guys.
as for carseats, i have taken my now 5 year olds to europe a few times. we never bring our car seats. just impossible to drag them around. when you're in a taxi you're in and out and no taxi will be waiting for us to load and unload. taxis are cheaper than renting a car.we never did rent a car.
you're in luck euro value going down, just 2 weeks ago was almost 1.4 to 1 with dollar and now it's down to 1.2 :). so follow that and if you see it creeping up go exchange some dollars now. i don't recommend using credit cards. i always use cash or traveler's check because cc exchange ratio is not the best, pus i am afraid for fraud.
anyways, happy traveling