A table of Hiragana/Katakana for learning the sounds.
The part of Duo that helps me most in learning the sounds is when you get the questions where they put the kanji in four cards and ask which one matches the hiragana they give you.
I always click on all four cards several times to help me get used to associating that symbol with that sound. It's so very useful!
I would really like to have a table with cards having the hiragana and katakana symbols and didn't have the romanji translating for me, so all I have to work with is the sound and symbols. Ideally, I'd like to be able to remove the cards once I had that sound down cold, so the sounds I needed to work on most would be easily found. The matching game almost does this, but you still have the distracting romanji there, which makes me want to use English pronunciation, which is not exactly the same sound the native Japanese speakers actually use.
On a brighter note, I'm noticing that I'm actually starting to associate some of the hiragana with their sounds now, makes me feel I'm getting to pre-school reading levels at least. :D