"I played a lot yesterday."
You bring up a very important issue, namely when Kanji vs Hiragana characters are normally used. I thought Kanji would be for all content words, whereas Hiragana is used for function words and conjugations, but you correctly inform us that it is used for adjs & advs as well. Even if we are dying to rid our dependency on Hiragana, you caution us accordingly - thanks.
昨日はたくさん遊んだ We need more informal forms. Once you know the ます form, you can always build all the other formal forms ました、 ません、ませんでした but the informal forms are harder to remember. Duolingo mostly accepts informal forms as valid answer (when the context does not show if it is formal or not) but never provides them and to me they were not wet introduced. I think they are important to know.