I have to disagree a little with the go for it on writing to your hearts content.

I freelance as a script reader and I've had tons of scripts where it seems that the writer did just that and created a total mess. Scenes were there that clearly the writer thought were cool or would be fun but were totally unnecessary.

So I will phrase the answer like this. If the 'touches' really add to the story, go for it. Don't worry about how it will be made. Just give enough to show the idea and the director will decide if it's CGI, practical effect, combo. If you are doing it just cause why not, don't. Specs are not the place for empty calories.

@Fan Girl: Perhaps I could have stated it more clearly, but I thought I made your point here:

If the story concept is a strong one, the characters are compelling, and the writing is good, a studio will not shy away from acquiring that type of script involving a minimal to moderate amount of CGI.

The presumption here is that whatever CGI sequences the questioner envisioned are not fluff or "unnecessary" because that's not good writing.

"Specs are not the place for empty calories." Absolutely agree.