Ways to say "It's OK" when someone says "Sorry" (Sumimasen)
 

If a stranger apologizes for something by saying "Sumimasen" how can you let them know that it's really no problem. For example, maybe they bumped into you, or spilled something on you, etc.

I believe I've heard you can say:
いいえ - (iie) - No. (which I suppose is a shortened version of "It's no problem" or "It's no trouble" or "No, it's not even worth apologizing for")

Would it sound strange to say:
いいです　 (ii desu) - It's good. It's ok. It's all right.
いいですよ　 (ii desu yo)
問題じゃありません (Mondai ja arimasen) - It's not a problem.

Just "OK" will work.

>いいえ - (iie) - No.
not natural

>いいです　 (ii desu) - It's good. It's ok. It's all right.
>いいですよ　 (ii desu yo)
>問題じゃありません (Mondai ja arimasen) - It's not a problem.

these will work well. You can also say "問題ない:MONDAI NAI;no problem" as a casual form.

Also, "大丈夫(です): DAI-JO-BU（DESU);It’s OK!" (add "DESU" for polite form) will work.

Great! Thanks!

What if you suffer a setback? For example, maybe you had plans to go to the beach with a friend, but it's raining.

Your friend says, "Looks like it will rain all day."

If you wanted to say, "That's all right..." (because you have a good book to read or something else you can do instead. You're not very troubled by the rain.) Would the same words/phrases above make sense? (understanding you probably wouldn't use DESU with a friend)