GPG offers symetric encryption of single files which can be integrated with most file managers.
Ususally this works in some way where you right click on a file and select "encrypt" from a context menu. This is common with dolphin and nautilus and there is probably some solution to do this in windows explorer as well.
Alternatively there are encryption layers for file systems. In GNU/Linux for example you could use fuse encfs, to have a folder encrypted. You can set up your dropbox client to only sync the encrypted folder while working on the mounted unencripted representation.
Even easier: encfs offers a reverse view: you can take an unencrypted folder and mount it to folder which then contains a virtual, encrypted representation of the files. Have this crypto view synced via dropbox and you wouldn't even change your workflow.