Some & Sum

As in, "You need to add some to get sum."

Beach & Beech

As in, "You may have noticed there was no post last week. Instead, I took a lake-y, forest-y vacation."

Tic & Tick

With a K – small, unwelcome parasite.
Without a K – small, unwelcome twitch.

Ad & Add

Two Ds – makes you smarter.

One D – makes you dumber.

Hey, it's my 200th post! And still no book deal! :P