Yes, it's Beauvais, not Charleroi (which is in Belgium) - at least it was a few months ago - but landing in Beauvais means a 60- to 70-minute bus ride to the Porte Maillot (I believe, or one of the Portes), and then you have to get yourself into Paris proper. Huge PITA IMO.

I would check easyjet and such airlines for dirt cheap fares, a few(maybe 2-4months) in advance.
For the museums, I book a week in advance(but the Alhambra in Spain maybe an exception, where you need to book in advance).

RyanAir give great deals from Paris to Dublin (for about 20 euro each) at times, but you have to look at their sale section every week or two!

Discount airlines are notorious for added fees - like seat reservations even on some - Ryan Air's boss even wanted to a few years back put pay toilets on his planes - that was shot down quickly but shows the strategy of added fees. Always ask what is the bottom line fare - everything you need included.

I wouldn't take the train from Italy to Paris. Fly Easy Jet from Venice or Rome (depends which city you fly into initially) to Paris Orly airport (closest airport). The night train isn't that great (there have been negative reviews on this board) and not all that convenient.

Since the train departs at 7:20 pm, you would have to arrange to store your luggage somewhere for the day after you check out of your hotel. Some hotels will store luggage for you, but then you would have to return to your hotel later in the day to retrieve it (might not be convenient). You could store it at the train station, but that would involve going to the train station in the morning just to store it. At times there can be lines at the luggage storage to drop off and pick up (you have to show ID, have luggage weighed, etc.). Also, unless you are willing to share cabins/couchettes with strangers, the overnight train is not cheap. A two person cabin is running 380€ from Venice to Paris.

Picking the same random date (Aug 24) that I picked for the train price, Easy Jet is 86€ for two people. They do charge extra to select your seat or to check luggage (they are very strict about carry on size/weight as are all the budget airlines in Europe). Flight travel time is 1:40. I think Easy Jet still releases their schedules about five months out. You can play around with some dates to get an idea of flight times and prices/fees.

http://www.easyjet.com/en/

>>>Since we plan to stay in Italy for at least a week, do you recommend renting a car? It seems like we are likely to stay in each city for 2-3 days before moving on.<<<

Rethink that schedule. Three nights somewhere only gives you two sightseeing days. Depending on city, many museums will close on a certain day. You will need to plan around that.

A car would be good if you planned to stay in the countryside (Tuscany?), but many cities in Italy don't allow you to drive into them (Google ZTL Italy and traffic cameras). Even hill towns have ZTL as do Rome and Florence. Parking in a bigger city would run 30-40€ a night (if your hotel has parking, many dont').

As for booking things, it's far too early. Train schedules in Europe change twice a year (2nd weekend in Dec and 2nd weekend in June) and Trenitalia allows booking fast trains 120 days in advance.

Skyscanner.com is a great website for european airfares. It's easy to play with the dates and destinations to help plan an itinerary. As mentioned, you have to be careful booking as the discount airlines charge for all manner of things: seats, baggage, check-in, etc. But, we have flown on a dozen different airlines in Europe with no problems other than the usual issues attendant to air travel. My one caveat is I have avoided Ryan Air (although my kids used them with no issues). Personally, I can't sleep on a train so an overnight does me no good, but day trips on modern and timely trains in France, Italy, Spain, Netherlands, etc. have been easy and fun.