Last night I was flipping through the John Adams biography by David McCullough. He frequently mentions Adams diaries. I don't know if the list is exhaustive, but you can see his entries (along with photos of the actual written page) at the Massachusetts Historical Society.
What's interesting is how mundane some of the entries are. Information that is perhaps unimportant is preserved where it would otherwise be unknowable. They're fun to read in part because of this.
A typical entry from his college days:
At Colledge, a Clear, warm morning, and so Continued.
During his time in France with Ben Franklin he often wrote:
Dined at home.
In a sense, these are the tweets and check-ins of another generation.
There is, of course, a lot more to find in his writings, but it's worth noting that keeping a journal doesn't mean having to write pages every day. Start, and perhaps finish, with a sentence.