Sometimes I ask a question for which the answer is very straightforward, and I missed the solution which is very obvious. I know that I can't delete a question after I got an answer. Is there something I can do about it (like recommending closing) or something like that?
Leave it open. If you missed the obvious solution, there's a very good chance that someone else will, too.
Sometimes I ask a question for which the answer is very straightforward, and I missed the solution which is very obvious.
That's not necessarily a problem. If the question is on-topic for the Stack Exchange site you posted it on, it doesn't really matter if it's 'easy' or 'hard'. Yes, you might get downvotes because users suspect lack of research - that's why you should always try to show what you've done / what you do know about the question.
Some sites in the network, like Stack Overflow, do have a close reason that might be applicable:
This question was caused by a problem that can no longer be reproduced or a simple typographical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers. This can often be avoided by identifying and closely inspecting the shortest program necessary to reproduce the problem before posting.
The 'simple typographical error' is such a problem with a straightforward solution, so in that case voting to close the question might be in order. But most sites don't have this.