It is prohibited to wear new or freshly laundered clothing, in order to show signs of mourning. This prohibition applies to men, women, and children who have reached the age of chinuch [i.e. can understand the concept of mourning]. This includes all outer clothing [e.g. shirts, pants], coats, towels, tablecloths, and bed sheets. Some opinions rule that this prohibition applies to undergarments. Harav Moshe Feinstein, zt"l (oral ruling, Shu"T Rivivos Ephraim 3:340) holds that one may wear freshly laundered undergarments. One may give fresh linen for a guest sleeping at his home (Laws of Daily Living, The Three Weeks, pg. 88).
One may wear any clothing that is not freshly laundered. Therefore, if one wore clothing before the Nine Days, he can wear it during the Nine Days. There is a dispute amongst the Poskim how long one needs to wear them. Harav Y.S. Eliyashiv, zt"l, holds for 30 minutes (Laws of Daily Living, The Three Weeks, pg. 85). Harav Shmuel Kamenetsky, shlit"a, holds that it is not dependent on how much time one wears them, rather as long as while he was wearing them he is not thinking about that he is wearing a freshly laundered shirt (Koveitz Halachos, Bein Hamitzarim, 11:27). Wearing many pairs of clothing at once does not help, unless he is sweated up (Koveitz Halachos, Bein Hamitzarim, 11:28).
There is a machlokes haposkim if one who did not prepare enough clothes beforehand, may switch his clothing [many times] on Shabbos [when it is permitted to wear freshly laundered clothing] so that he may wear those clothing during the week. Harav Shmuel Kamenetsky ,shlit"a, holds one may do so and it is not considered hachanah (preparing on Shabbos for weekday) because he is benefiting from wearing the clothing at the time (Koveitz Halachos, Bein Hamitzarim 11:29).
Another option available, is to place his clothing on a floor which is not freshly cleaned [but does not have to be definitely dirty] (Koveitz Halachos, Bein Hamitzarim 11:30). Additionally, one may step on his clothes or sleep on them, or place them together with dirty laundry [e.g. in a hamper] to make the clothing not fresh (Laws of Daily Living, The Three Weeks, pg. 87).
Bathing, swimming, and showering for pleasure, even in cold water, is prohibited. Refraining from these activities and being a little uncomfortable is in order to remind ourselves about the destruction of the Bais Hamikdash (Shulchan Aruch & Rama 551:16).
One who showers daily, and finds it very difficult not to shower, maypossibly take a non-hot shower with soap (Koveitz Halachos, Bein Hamitzarim, 14:4). One does not have to have an exclusively cold shower; one may add some hot water in order to take away the chill. One who normally takes a hot shower every Erev Shabbos (Friday) may take a hot shower on ErevShabbos of the Nine Days (Halachos of Three Weeks [Rabbi Shimon Eider, zt"l] pg. 13, # 7).
Swimming for pleasure is prohibited even for very young children (Laws of Daily Living, The Three Weeks, pg. 96). However, they may play in a sprinkler (Koveitz Halachos, Bein Hamitzarim, 14:3).
Construction or decorating should be postponed until after the Nine Days (Shulchan Aruch 551:2). Building for basic dwelling purposes, to prevent damage, and basic upkeep of a house or for a mitzvah is permitted. Included is planting for pleasure, or buying plants for decorative purposes. However, planting, etc. for regular upkeep is permitted (Laws of Daily Living pg. 97- 98).