There are currently 107,156 graphic characters in Unicode, which include the keyboard characters. All of these can be used in normal text.
The viewer must have at least one font on their system in which that character appears in order to see it. Otherwise it will appears as a blank box or as a question mark or something similar. Accordingly, if you are using symbols, it is advisable to choose symbols which are in a number of common fonts if you wish your symbols to be seen.
To see all the characters available, look in the official Unicode charts at http://www.unicode.org/charts/ . Most of them also appear in http://en.wikipedia.org/wiki/List_of_Unicode_characters and its links. Each Unicode character has a unique code in hexadecimal that can be used to identify it. The code can also be translated into a decimal code using a programmer's calculator.
HTML entities may be created from these codes. For the hexadecimal HTML entity, one types & # x without spacing followed by the hexadecimal code followed by a semi-colon (;). For the decimal HTML entity, one types & # without spacing followed by the decimal code followed by a semi-colon (;). In some forums, such as this one, the HTML entities can be typed into your post and will be converted into the corresponding Unicode character. This forum is one of those where the entities work. They are switched to the corresponding character when you mouse-press “Preview”.
Use the site http://www.fileformat.info/info/unicode/char/search.htm when you have a character for which you do not know the codes. Either paste the character itself into the Query box, or enter its hexadecimal code, or its decimal code, or all or part of its official name, or all or part of one of its official alias names.
Most of the base plane Unicode characters work on this site. If more than one answer appears, click on the one you want. If only one answer appears, click on it. If no answer appears check your typing.
An information page will appear showing a graphic or the character, giving its HTML entities, and providing further information, including a link to a page listing common fonts which contain the character. The second page also provides a link to a tool which lists what the character looks like in all Unicode fonts which you have installed. (For some characters most or all of these fonts will show only a box character, indicating you do not have the character in a Unicode font.)
You have not indicated what computer operating system you are using. I suggest you consult the site http://ipa4linguists.pbworks.com/ which is intended to tell linguists how to generate Unicode phonetic characters under the Windows, Macintosh, or Linux operating systems. But the methods used mostly work for any character.
The Alt-character method suggested by some only works as described on Microsoft machines and only for those characters which are in your current DOS font and in your current Windows 8-bit font. These character sets vary somewhat from machine to machine, depending mostly on the default language, about which many websites appear to be naively ignorant. The characters from 0 to 127 and the characters from 01 to 0127 will work the same on any machine. But this only allows access to a very small subset of what is available from Unicode.
There is an alternate Alt-code method used in some Microsoft products like Notepad and Office which extends the normal Alt-code method to characters greater than 255. You can, in this method, obtain the character for any base plane Unicode character.
You can also download and install the Quick Unicode Input Tool from http://www.cardbox.com/quick.htm which will allow you to enter the Alt key plus numeric keypad decimal code or numeric keypad dot followed by the hexadecimal code in any application. (Note that when this is running it replaces the normal Alt-key method, and so the Alt-key DOS codes are not available. You must enter the Unicode values for the characters.)
For various other Unicode Utilities see http://www.alanwood.net/unicode/utilities.html .
For the peace symbol (☮), see http://www.fileformat.info/info/unicode/char/262e/index.htm .
For flowers, see (⚘ ❀) http://www.fileformat.info/info/unicode/char/2698/index.htm and http://www.fileformat.info/info/unicode/char/2740/index.htm .
For smiling symbols, see (⌣ ☺ ☻) http://www.fileformat.info/info/unicode/char/2323/index.htm and http://www.fileformat.info/info/unicode/char/263a/index.htm and http://www.fileformat.info/info/unicode/char/263b/index.htm .
For check signs, see ( ✓ ✔) http://www.fileformat.info/info/unicode/char/2713/index.htm and http://www.fileformat.info/info/unicode/char/2714/index.htm .