As a day trip from Paris, Giverny is a manageable task.    I suggest taking a train from Paris Gare St. Lazare to Vernon.  It's about a 45 minute train ride.   Take the early train, at  8:20 arriving at 9:05.   I also suggest purchasing train tickets in advance or online, they cost about 30 euros round trip.

Upon arrival in Vernon, it is in your best interest to get to Giverny, about 5 miles, from the train station before the big crowds roll in.   Be quick and grab one of the few taxis that will be waiting at the station.  Otherwise, there is a public bus, at a cost of about 4 euros, that will deposit you about a quarter mile from Monet's house, but probably not in time for the 9:30 opening .  If taking a taxi, make arrangements for pick-up after your visit is completed.

Another advantage of taking a taxi is that the drop off point is right at the entrance to the home/gardens.    There are also bicycles to rent at a cafe adjacent to the train station and I have personally done the 5-mile trail to the home/gardens.

Upon paying the 9.50 euro fee, I head straight towards the back to the lily pond in the hope no one else has arrived yet.  This is the opportunity to take the best photos while not being interrupted by others.   Afterwards, head back towards the house and visit the home and studio of Monet.  I was pleasantly surprised last week to find that no-flash photos can be taken inside the house now.

I think a tour of about 2-3 hours is perfect.   It's going to be necessary to coordinate departure time with train schedules back to Paris.   However, there are a few nice places to enjoy lunch very close to the train station while waiting for your ride back to Paris.