Himeji Castle

I will be in Okayama April 28 and 29. I had planned on visiting Himiji Castle, but I have been told it is closed for renovation. Is it completely closed, or are parts open? Should I skip Himiji Castle and visit Okayama Castle instead?

The interior will be closed and the exterior will be covered by scaffolding. But the rest of the grounds are still open. If you want to actually see and enter a castle, you better visit a different castle. Okayama is not bad, but it is a modern reconstruction and its interior is a modern museum rather than an original castle.

I'm planning to go Himeji Castle on 7 Apr from Osaka. Is the castle already covered by scaffolding? Or is that going to happen only in mid Apr?

You should be good as the renovation project will commence April 12.

Read this for more details
http://www.japan-guide.com/e/e3501.html

I assume you are already planning on visiting Korakuen Garden in Okayama. It is right by the castle, so you can actually see the castle with the garden.

If you are looking for another authentic castle, Bitchu Matsuyama Castle is within daytrip distance. It's in Takahashi (still in Okayama Prefecture). It's the only remaining original mountain castle. It's much less touristy than Himeji, but still highly impressive (although nowhere near as large).

No, Himeji Castle will not be covered up yet. According to a recent JNTO article, it will actually take months until the castle tower will disappear under scaffolding completely.

You can expect both, an extremely beautiful sight of Himeji Castle with cherry blossoms on April 7 as well as a lot of other tourists who want to see the castle before its long renovation. Especially if the weather happens to be good, you should anticipate line ups.