Definitely the gimme. Either a slingshot around the sun or the Guardian of Forever would bring the USS Enterprise crew to Gotham. You've probably seen the fan-made YouTube video where Kirk and Spock mix it up with Batman, Robin and The Joker.
Batman '66 has already used time travel with King Tut to head back to Egypt. Why not use it for Shame to head back to the Old West...Well, the steampunk Old West
"S" as in "splendid", "M" as in "marvelous". Batman '66 was played straight - for laughs. Get Smart was Mel Brooks' parody of the Bond spy craze. A craze that included Derek Flint, Matt Helm, The Man From UNCLE, The Girl From UNCLE and I Spy, just to name a few. This would be epic!