How concerned should I be with these *toxic* items?

Over the past year I've come to learn how many common household items are either toxic or not good for cats. I'd love some more insight into whether there are real concerns surrounding things like Onions, Garlic, Onion/Garlic Powder, Lemons, Mint, or even things that contain oils such as Lavender. I live in a household with people who aren't always so great at cleaning up after themselves and/or being aware of their surroundings, and in turn have become a bit worried one of the above (or other items) could be left to be later found, eaten, or stepped on and accidentally ingested by my cat. I'd just love some light shined onto if these are real concerns, or if I have no real worry. Thank you!