Pamela Hovland alerted me to these cool jars that have chalkboard labels so you can just scribble the contents (or the date) with chalk. You can by them at Rockett St. George for 12.50 euros and have them shipped from the U.K. OR you can rig your own. Paint labels directly on jars using chalkboard paint by hand or with chalkboard spray paint (follow the directions and caveats here). Or make or buy press-on labels like these from an industrious Etsy seller who cuts them out of chalkboard vinyl (fine for jars with a flat surface).
The wonderfully-shaped Weck canning jars would be great with painted labels (though press-on labels would probably work on their gentle slope.