Draw a hollow square of # with given width

Charcoal, 6 bytes

Code:

ＮβＢββ#

Explanation:

Ｎβ        # Get input from the command line and store into β
   Ｂ      # Draw a hollow box with...
     β     #  Width β
      β    #  Height β
       #   #  Filled with the character '#'
           # Implicitly output the box

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MATL, 12 bytes

:G\1>&*~35*c

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Explanation

:     % Input n implicitly. Push range [1 2 ... n]
      % STACK: [1 2 3 4 5]
G     % Push n again
      % STACK: [1 2 3 4 5], 5
\     % Modulo
      % STACK: [1 2 3 4 0]
1>    % Does each entry exceed 1?
      % STACK: [0 1 1 1 0]
&*    % Matrix with all pair-wise products
      % STACK: [0 0 0 0 0;
                0 1 1 1 0;
                0 1 1 1 0;
                0 1 1 1 0;
                0 0 0 0 0]
~     % Negate
      % STACK: [1 1 1 1 1;
                1 0 0 0 1;
                1 0 0 0 1;
                1 0 0 0 1;
                1 1 1 1 1]
35*   % Multiply by 35
      % STACK: [35 35 35 35 35;
                35  0  0  0 35;
                35  0  0  0 35;
                35  0  0  0 35;
                35 35 35 35 35]
c     % Convert to char. 0 is interpreted as space. Display implicitly
      % STACK: ['#####';
                '#   #';
                '#   #';
                '#   #';
                '#####']

Python 2, 62 54 bytes

f=lambda n:'#'*n+'\n#%s#'%(' '*(n-2))*(n-2)+'\n'+'#'*n

Returns #\n# when the input is 1

55 Bytes version that prints

def f(n):a=n-2;print'#'*n,'\n#%s#'%(' '*a)*a,'\n'+'#'*n

62 Bytes version that works for any input:

f=lambda n:'#'*n+'\n#%s#'%(' '*(n-2))*(n-2)+('\n'+'#'*n)*(n>1)