I know certain agricultural activities are prohibited on Chol HaMoed -- it's not the recommended time to do gardening or lawn-mowing (if there's sudden pressing need, talk with your rabbi). What about putting cut flowers into water? Is that "agricultural work"? (I know it would be prohibited on shabbos ...)
Shulchan Aruch (Orach Chaim 336:11) permits putting branches into water on Shabbos, "as long as they don't have buds or flowers that will open because of the moisture from the water." Shulchan Aruch Harav (ibid. :18) and Mishnah Berurah (ibid. :54) clarify that this means putting them into a vase that already had water in it, but that one may not fill a vase on Shabbos and put the branches in it.
Elsewhere (654:1) the Mechaber adds that on Yom Tov water may even be added to the vase in which the lulav is kept, but that the water may not be changed; "but on Chol Hamoed it is a mitzvah to change the water so that the lulav stays fresh and beautiful."
Now it is true that a lulav doesn't have flowers on it. Nevertheless, if you're allowed even to change the water on Chol Hamoed and that's not considered "too much trouble" (טירחא יתירא), then it would seem logical that you can do the same for cut flowers, and that it wouldn't matter whether the flowers are going to open or not - it should be no worse than watering a field that needs it, which is permitted on Chol Hamoed (ibid. 537:1).
I'm not a Rabbi and this is not Rabbinic advice, BUT:
I'm certain that putting cut flowers into water is permitted on Chol HaMo`ed. Flowers presumably enhance one's enjoyment of the holiday, which would indicate that one can obtain them and put them in one's house. And if they're not put in water, they will wither and die, so not putting them in water would cause a loss.
To me, it doesn't sound like "work" at all, either, and takes no skill or effort to do.
The Rambam (Sh'vitat Yom Tov 7:20) permits all forms of personal makeup and beautification on Chol HaMo`ed, and putting flowers into water is, I think, similar but with less work.