Ted Cruz REFUSED to endorse Trump at his own convention... What's that guy up to??

While I cannot cite any particular evidence to support it, I think your guess is a good one. Cruz setting himself up as the "anti-Trump" to lead the GOP back from the 2016 pasting it is likely to receive (if current trends hold) seems to be a likely explanation.

You're right he is definitely planning to run again. He should have endorsed Trump.

Who knows.  Only time will tell.

It is the only brilliant and brave move this guy has ever enacted.   I am proud of him and I don't even like him.  That took guts.  GOOD FOR HIM. GREAT FOR HIM.  It was an excellent ploy.  My estimation of him went up.

He has absolutely NOTHING to gain by backing Trump except embarrassment.   He has everything to gain by standing against him and standing his ground.  It also served to underscore how shaky the Trump ground is, how the unification of the Republican party under Trump is a lie and a joke. 

Ted Cruz announced to his Texas delegation that he was not a "servile puppy dog" who could support a candidate who attacked his wife and father. So he wouldn't endorse him. But he gave an excellent speech on conservatism.

Trump had read his speech a couple of hours before the convention so he knew about it and allowed him to speak.

Ted did congratulate Trump in his speech. And Trump did invite him to speak. At least he showed up. Look at the ones who didn't!