Basically any skill that was essential before technology.
Things like the OP said, remembering phone numbers, mental maths, manual filing, legible handwriting, the manual methods of work and even survival.
Although I take your point, and personally regard those skills as a given, I'm not sure any of those skills are necessarily forgotten, or being eroded. Technology may render them less useful, but go back 50-60 years, and remembering phone numbers was only just becoming a needed skill. Go back 100 years, and the same could be said for handwriting. In general, the population of the UK in particular has not needed survival skills for probably a hundred years, whilst mental maths is primarily limited to simple equations, as in the example given earlier.
I honestly don't believe that any more mental agility is required by mental maths, memory use, or manual work, than is required to operate the myriad of different controls on a PS3/Eggbox 360 controller. Our lad can quite happily multitask on a PS3 with a familiarity and skill that I will never master, but, no, he wouldn't be able to give me an instant answer to 15% of 80.
Whilst both distinct skillsets might be different, they are still learned, and I don't believe the technology is the problem. I would say that a grasp of computers, The Internet, Twitter, Facebook is just as essential today, as remembering phone numbers was in the 50's to 80's.
However, go live in a village in Africa, and different skills become essential. So, in answer to the OP, no, technology has not affected mental agility, merely the skillsets that are learned.