Ten million pennies:
Type $@! during game play. Note: This code may only be enabled a total of five times.

Hidden monster:
Type SAMNMAX three times (case-sensitive), and ignore the windows that appear in response.

Secret building:
Change the view to north in Hell. Locate an unused corner and build a square road surrounding an empty 7x7 space. Change time mode to "Divine Intervention". Build the following pattern in that space, where B is Blue, BR is Brown, O is Orange, G is Green, P is Purple, Y is Yellow, and R is Red:

    G  Y  O  BR P  R  B
    Y  O  BR P  R  B  G
    O  BR P  R  B  G  Y
    BR P  R  B  G  Y  O
    P  R  B  G  Y  O  BR
    R  B  G  Y  O  BR P
    B  G  Y  O  BR P  R