I think for German, new syntax cards would work. I'd do the following:
With an example sentence like this:
Er gibt einem Mädchen einen Kuss.
He gives a girl a kiss.
Blanked out sentence: Er gibt __ einen Kuss. [picture of girl]
Back(Word): einem Mädchen
Root Word: das Mädchen
Always Back: einem –> Dativ (masc./neutral)!
And do the root word tests
Then I’d get 4 cards, two asking me about the declined form (einem Mädchen), and two asking me about “das Mädchen”, all with a note on the back reminding me that “einem” is Dative.
In addition, I’d set up some mnemonics for the cases and make some flashcards to remind me about them. In tinkering around with various imagery, I’ve found that I like using gestural mnemonics for noun cases.
[STANDING ON OBJECT] = nominative
[POINTING AT OBJECT] = accusative
[HEADBUTTING OBJECT] = dative
[GRABBING/CRUSHING OBJECT] = genitive
Then if I see a sentence like:
Er gibt __ einen Kuss.
And I get the case wrong, then I can re-imagine the story in the sentence with the guy headbutting a girl, and then kissing her, to help me recall the proper case we’re after.
Same thing if I can’t remember which case “einem Mädchen” is in when I see it on the comprehension card. If I see that phrase and I can’t remember what case it’s in, I’ll think about imagery of people headbutting girls.
[Apologies for the violence, it’s just how I store things!]