This is a nice resource from Erich Friedman, a math professor at Stetson University: it’s a list of distinctive characteristics of [most of] the numbers between 1 and 9999:
http://www2.stetson.edu/~efriedma/numbers.html
Now, maybe knowing that 215 is equivalent to 555 in base 6 isn’t that useful, but there are a lot of great ideas woven throughout this list of integers. If you can fill in any of the gaps (do you know anything distinctive about 6821?), I’m sure Dr. Friedman would love to hear from you.