Learn Hardcore Japanese: hanami no ato de hakubutukan ni mo yotta node, sono hi ha itinitizyuu yoku arukimasita. - After cherry-blossom viewing, I also stopped by a museum, so I walked a lot that whole day.

Breakdown of hanami no ato de hakubutsukan ni mo yotta node, sono hi wa ichinichijuu yoku arukimashita.

Why is there a の after 花見?

In 花見のあとで, the の links the noun 花見 (flower viewing / cherry-blossom viewing) to あと (after).

So:

This is a very common pattern:

When a noun comes before あと, you usually connect them with の.

What is the difference between あと and あとで here?

あと means after or later, and で often makes the phrase function more naturally as a time expression in the sentence.

So:

In many cases, のあと and のあとで are both possible, but あとで often sounds a little more like after that, then...

Here, 花見のあとで博物館にも寄った gives the sense of: After the flower viewing, we also stopped by the museum.

Why is the particle に used with 博物館?