The Shulhan Aruch (336:10) rules that it is permitted to smell a Hadas (myrtle) bush, whereas it is prohibited to smell fruits growing on a tress, such as apples or Etrogs. The difference is that the fruit is edible. The Hachamim were concerned that smelling fruit may lead one to pluck the fruit off the tree, which is a Torah prohibition. The Hadas is meant only for smelling, and therefore there is no concern that it will lead to picking. Based on this principle, it is permitted to smell any fragrant flower or bush on Shabbat.
The commentaries disagree as to whether the branch is Mukse and may be held with one's hands while he smells it. The Bet Yehuda (1:11) argues that since the Shulhan Aruch did not stipulate that one should avoid handling the branch, it can be derived that it is permitted. His leniency is grounded on the assumption that "En Mukse B'mhubar"-Mukse does not apply to that which is rooted in the ground. This is also the opinion of the Meiri (Succah 37b), the Ri"az and the Nahar Shalom. Although there are some dissenting opinions who are stringent (Taz), Hacham Ovadia (7:36) and Menuhat Ahaba (Rabbi Moshe Halevi, Israel, 1961-2001, Ch. 13) conclude that one may be lenient.
This issue touches on another question, as well. If one is sitting on the grass on Shabbat, may he touch and move the grass with his hands? If Mukse does not apply to something that is attached, there would be no problem, as long as he is careful not to uproot the grass. The Aruch Hashulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908, 336:46) is clearly lenient.
However, this would not apply to someone who goes to the beach. Sand is Mukse "Mahamat Gufo,"- intrinsically excluded from use on Shabbat. While it is permitted to walk and sit on the sand, it may not be handled on Shabbat, since it is not attached to the ground. Therefore, one should avoid beaches on Shabbat. Sand which is in sandbox is not Mukse, since it was designated for play (Hacham Yishak).
SUMMARY
It is permissible to smell flowers attached to trees, and they may be held with one's hands.
It is permissible to touch or move grass, as long as one is careful not to pull it out of the ground,
Sand on the beach may not be handled on Shabbat.
