Your standard call-to-action post will include words like share, retweet, or comment. The point of doing this is to get people to perform that action via your post, content, or brand. The action must be doable. Your post won't get any interaction if users aren't able to execute it--so think about it, and test it before putting it out there.
One great call-to-action hashtag is #ShareYourEars. On Twitter, Disney partnered with the Make-A-Wish Foundation. They created a campaign encouraging everyone to share a photo of them with their Mickey ears on. For every photo tweeted, Disney donated $5 to the foundation. It's hard not to contribute when it's for such a good cause. This also reinforced Disney's brand image of being compassionate and willing to give back--and they raised a staggering $3 million.
Tip: Having a core message will win over most demographics; everyone loves a good cause. Make it fun and easy to do.