Not sure when your trip is (or was), so I might be too late to this party, but here are my two cents.
I agree with others who say that combining Giverny with Rouen is too much in one day. If you want to make a full day of it in Giverny, you might be interested in including a visit to Musée des Impressionnismes Giverny. It's a lovely museum just down the street from Monet's house. It's small (3 galleries), so not too overwhelming. You'll find a description of museum in the RS Paris guidebook (Paris day trips chapter for Giverny). More info here: https://www.mdig.fr/en/
I see that the current exhibit (through June 29) is titled "from Monet to Picasso." A different exhibit opens July 11.
In June 2023, we were able to visit this museum late morning and had lunch at their nice cafe. We saved our visit to Monet's house until later in the afternoon as others had advised (4-6 pm), when the light is lovely and the crowds start to thin. The other advantage to visiting the museum first is that we could buy a combo ticket to the Museum + Monet's house. Check the fine print to see if a combo ticket is still offered. If you are even remotely interested in Impressionism, I would go to this museum in a heartbeat.
If you do not want to go to the museum, you'll want to reserve tickets in advance for Monet's house, as others have noted.
And, I see from your original comment that you were not interested in museums when contemplating things to do in Vernon. So, I make this suggestion with caution.
Agree with Pam's suggestion to visit the church and burial sites of Monet and the Lancaster D-Day bomber crew. (Shout out to Pam for recommending this to me 2+ years ago!)