I took Eric Foner's edX Civil War class and he mentioned Seward had a plan to have the Union try to take Cuba from Spain, and that this would force the South to join together with the North for a common cause. Is this true? Was this just an idea of Seward's or a serious proposal?
Seward had a number of schemes to provoke a foreign war in order to force the South to abandon secession. The Disunion column in the times covered the conspiracy you're talking about here: http://opinionator.blogs.nytimes.com/2011/03/17/the-foreign-war-panacea/?_r=0
He also infamously wrote a memo to Lincoln in 1861 urging that the administration should drum up a conflict with one of several European powers in order to "rouse a vigorous continental spirit of independence on this continent against European intervention."
I didn't have any luck finding a full version of that memo online, but you can find selections here: http://blogs.dickinson.edu/hist-282pinsker/files/2011/08/Handout-Sewards-memo.pdf
If you're interested in Seward and his influence, I'd strongly recommend this biography: http://www.amazon.com/Seward-Lincolns-Indispensable-Walter-Stahr/dp/1439121168/ref=sr_1_1?ie=UTF8&qid=1416972239&sr=8-1&keywords=seward