I support Rekmeyata's suggestion of titanium if you want something different. That said, i just started negotiation to buy a Chinese carbon frame with the intention of using the finished bike about the way you plan to---on days when I feel like going rapidly rather than long, and whenever lugging extra weight uphill might make things harder than they already are.

If I ever get another bike after this, it will definitely be titanium though ... a better ride than steel, even, from what i hear, no corrosion, and more crash-worthy than CF (though I have recently learned that aging flesh is not particularly crash-worthy so the bike frame material might not be an issue ... )