how do you form informal questions with 何 and without ですか？ would 何 just go at the end?
「あなたの大好きな季節何？」
and is it appropriate to leave off です or だ with な adjectives and nouns? i’ve seen 〜なの？used, but i’m not quite sure what the rules are on that.
「これは食べものなの？」 vs 「これはたべもの？」
basically i’d like some guidelines on informal questions in japanese! i keep struggling on what sounds normal because i don’t want to be overly formal and use ですか？all the time!
There are a lot of ways you could go with this. Informal language tends to be a bit different between males and females, for one. In your first example,「好きな季節は？」or「どの季節が一番好き（だ、なんだ、なの）？」would work. Here, だ is more masculine-ish, なんだ is also somewhat masculine-ish and softer, なの is feminine-ish. You also probably wouldn't want to use 大好き here, since you presumably don't know if the other person has a season they like that much. It is possible to say e.g.「僕の大好きな季節はな～んだ？さあ、当ててみて」though.
「これ、食べもんなの？」,「これは食べ物なのか？」,「これは食べ物か？」are all pretty normal ways to ask the question.「これって、食べられるの？」「これ、食べられるか？」are similar ways to ask the same question (well, they literally mean "is this edible?" though).「これはたべもの？」can certainly work (it can sound cute/funny), but if you don't want to sound like a foreigner you probably don't want to use it.
For some formal → informal examples:
「どこへ行きましょうか？」 → 「どこへ行こうか？」 ⇔ 「どこ行こうか？」
「何をしているんですか？」 → 「何をしてる（の）？」 ⇔ 「何してんの？」
「それ、やめてくれませんか？」 → 「それ、やめてくれる？」
「どうですか？」 → 「どう？」
「これでいいですか？」 → 「これでいい？」
