This is a little off topic because it's not searching by categories but I find it useful to sometimes just search by a database. For example over the Memorial Day weekend Fold3 was free for a few days. Ancestry has a database entitled U.S. WWII Draft Cards Young Men, 1940 - 1947. Fold3 has the same database only their database includes the images of the cards. So I use the search string to just search the database on ancestry and the cards were available to copy. I didn't get fancy. I copied all the cards to each individual person then after the fact went back created sources and facts etc.
For those of you who don't have the search string it is currently:https://www.ancestry.com/hints/tree/155115149/hints?hf=recor...
Change 155115149 to the number of your personal tree on ancestry because the 155 number is MY tree. If you use that number will bring up the hints for my tree not yours. The number 2238 at the end of the string is for the World War II draft cards database. Each database has its own unique number which you can usually find in the address bar for that database.
For example, 60525 is the number for the find a grave hints.
Looks like they cut off the string after the word hints? so also type after question mark hf=record&hs=last&hdbid=2238