Korakuen garden is way beyond "nice"! I have been looking at Japanese gardens for over 20 years and only made ot there this month. I think it is one of the most beautiful and fascinating that I have ever seen.I was also very impressed with the castle museum, and found that Okayama has other cultural sites of very high value. Also, it is a nice town for wandering in.
And tht's just the city. In every direction there is something interesting or beautiful: Bizen, Kurashiki, Naoshima and other islands (eg Hinase), Kibi, all of the International Villas. Hiroshima is not far in one direction; Himeji in the other.
In all, I think the area is one of the great overlooked zones of Asia.
But as for "day trip to Tokyo", that is a very optimistic notion, unless you have a specific meeting there or something.