Paige, there are numerous options. It all really depends on what the circumstances are. Is it a business trip? How important is it to get there quickly.
Buy a plane ticket: fast, easy, fairly no-nonsense.
Buy a train ticket: longer, relaxing, pretty simple.
If he's actually vacationing, night trains are nice. There is a train from Paris to Milan that leaves at 8:30 pm and arrives in Milan at 5:38 am.
The night train to Venice is the same train, and arrives there around 9:30 am.
Plane tickets are fairly straight forward and I would be pretty surprised if you can find a one way ticket to either MIlan or Venice from Paris for under $100, especially if your husband is flexible on departure.
Train tickets are sometimes hit and miss, especially on night trains. If you book far in advance, you can get fairly good deals, and in some cases pay less for a bunk than you would for a hotel room.
If you want to buy a train ticket, I would suggest the SNCF website (voyages-sncf.com), which is in French. You can use Google Translate to get the key information from the site if you're not able to understand French :) I have found that when you try to buy tickets through SNCF, and actually tell them that you're from the US, they kick you over to raileurope.com, which seems to me to be price gouging terribly. From what I've seen, I would NEVER use raileurope.com
If you choose to buy train tickets on voyages-sncf.com, you can select to pick up the tickets at a train station. Then all you need is your booking ID/confirmation number thingy and they will give you your ticket.
I assume you can handle the logistics of booking a plane ticket. Use kayak.com, if you're not already.
Geoff