I love the nickname [name_u]Cal[/name_u], but what are some full names that could use this as a nickname? I do like [name_m]Calvin[/name_m] but it’s relatively plain compared to my usual taste. [name_m]Caleb[/name_m] is totally unusable to me. Are there any more daring/extravagant options? Or really, any options besides these two names would be welcome.
I love [name_u]Cal[/name_u]! Personally, I would consider just [name_u]Cal[/name_u]. If you want a longer name, how about [name_m]Callen[/name_m]/[name_m]Callan[/name_m]?
McCallister may be an option.
[name_m]Callum[/name_m] is another option.
I [name_u]LOVE[/name_u] the name [name_u]Cal[/name_u]!!
What about [name_m]Malcolm[/name_m] or [name_m]Callahan[/name_m]?
Also:
[name_m]Callum[/name_m]
[name_m]Calvin[/name_m]
[name_f]Caledonia[/name_f]
[name_m]Clarence[/name_m]
[name_m]Callaway[/name_m]
[name_m]Calix[/name_m]
[name_m]Calhoun[/name_m]
[name_m]Pascal[/name_m]
[name_u]Calder[/name_u]
[name_m]Calloway[/name_m]
[name_m]Caleb[/name_m]
[name_m]Callen[/name_m]
Unfortunately, I’m right there with you on this one. I adore the nickname [name_u]Cal[/name_u] but I’m not in love with any of the full name options. I’m okay with [name_m]Calvin[/name_m] and [name_m]Callahan[/name_m] but not quite sold on either one. 
I think the previous posters hit most of the options. I can only think of a few others:
[name_m]Calix[/name_m]
[name_m]Pascal[/name_m]
Caldwell
[name_m]Caledon[/name_m]
[name_u]Campbell[/name_u] (maybe a stretch)
The only thing i can think of for [name_u]Cal[/name_u] that hasnt been mentioned yet is Kalten (my brothers name. its fictitious, from a book, the character was a knight) Its easily shortened to [name_m]Kal[/name_m] as a nick name and its very different. [name_m]Ive[/name_m] never heard another kid with the name. And you could certainly play with the spelling if you prefer [name_u]Cal[/name_u] with a C.
The first time I heard [name_u]Cal[/name_u], I fell in love with it because of the fictional character it symbolized. His real name was [name_m]Calhoun[/name_m] which is, of course, incredibly masculine and fun, but definitely one any little boy not living in the Victorian age would have to grow into! Therefore, on your part, it would be daring, for sure!
Since then, I know the mother of a little boy (about 7 yo) named [name_m]Callahan[/name_m], which [name_f]IMO[/name_f] is extremely chic and classy.
I adore [name_m]Calendar[/name_m]. Nicknames include [name_u]Cal[/name_u], [name_m]Len[/name_m], [name_m]Lenny[/name_m], [name_m]Dario[/name_m].
I love [name_u]Cal[/name_u]!
These have been mentioned already, but imo your best choices are…
[name_m]Callum[/name_m]
[name_u]Calder[/name_u]
[name_m]Callahan[/name_m]
[name_m]Calloway[/name_m]
A favorite unusual name of mine is [name_m]Caledon[/name_m]. And if I ever name a son that, his nickname would be [name_u]Cal[/name_u]. The only downside I see is that it’s the antagonist’s name in Titanic and I always associate it with that.
I love [name_u]Cal[/name_u], but prefer it on a girl. For a boy, full names could be: [name_m]Callum[/name_m], [name_m]Calvin[/name_m], [name_u]Calder[/name_u], [name_m]Callahan[/name_m], Caleo, [name_m]Calix[/name_m], [name_u]Calen[/name_u], [name_m]Calixte[/name_m], [name_m]Callaway[/name_m], [name_m]Caledon[/name_m], [name_m]Calbert[/name_m] and [name_f]Calais[/name_f].
I adore the nickname [name_u]Cal[/name_u] as well. My favorite boy’s name for it is [name_m]Callum[/name_m], though I also love [name_f]Callie[/name_f] for a girl. Second choice for a boy is probably [name_m]Callaghan[/name_m].
I think other posters have hit most of the options, and if you were looking for something else, you could just run a search for any name containing cal and find more choices. However I wonder if you might consider picking something that might be a bit different (after all, the very similar [name_m]Hal[/name_m] is short for [name_m]Henry[/name_m], which isn’t so obvious). I think you could easily make [name_u]Cal[/name_u] a nickname for [name_m]Charles[/name_m], [name_m]Carl[/name_m]/[name_u]Carlisle[/name_u]/[name_m]Carlson[/name_m]/Carmichael, [name_m]Caius[/name_m], [name_u]Campbell[/name_u], [name_m]Camilo[/name_m], Malcolm, or pretty much whatever you want. I think I actually really like it for [name_m]Charles[/name_m], even though it doesn’t even share the hard C.