You may want to start with these examples, then try your own. You can also get some ideas by looking at the fascinating wordcloud.
Click below to go to the live charts:
Union - Concern about the strength of the federal union peaks in 1800's until the union becomes more stable and secure after the civil war.
Women - Women are not mentioned at all in inauguration speeches until the early 1900's, leading up to women's right to vote.
Hope - This percentage plot of "hope" shows that Lincoln and Kennedy were the most "hopeful" presidents, followed by George W. Bush.
Constitution - The constitution was mentioned more frequently in early years, as its authority was still being established and ammendments were added. In later years, the constitution was mentioned less, perhaps since it's authority was fully established and fewer ammendments were added.
Freedom - Use of this term increases over time, peaking with G. W. Bush.
Foreign, World - In the early years, Presidents used the term "foreign" more frequently, perhaps since America was young and isolationist. In later years, there is greater use of the terms "world" and "international", perhaps indicating that Presidents are more engaged with other countries around the world.