I have a few friends that tell me honestly when they're too tired or simply not in the mood to spend time with me. It's a bummer for me, sure. I don't like to hang out with a lot of people so when they cancel it's a little sad. But I don't take it personally. I wouldn't want somebody to feel forced to hang out with me after a long day if they're not in the mood.
I think that lying to save face or not to hurt feelings is detrimental to our society in the long run. I think so much of our lives has become based on misleading people and hiding the truth on a daily basis, and while it's just the way of things, we'd be a lot more comfortable with ourselves if it wasn't the case.
I'm sure you already know this, but the brain is an insanely complex thing. When left to our imaginations, we will basically always come up with something far scarier than the truth. That's why when you're torturing somebody, you don't tell them what you're going to do. You let them imagine what you can do, because what they imagine will often be outside the realm of possibilities, but that's a tangent (and let the record show that I definitely don't know anything about torturing people). So when somebody lies to you, and you know they're lying, most likely you'll imagine a truth that is harder to accept than what is actually happening.
I can easily conceive a scenario where a significant other A lies about where they were that day. Person B could assume they're being cheated on when really, person A just went to hang out with a friend B doesn't like or went to do something as a surprise for B. It doesn't even have to be malevolent!
Now I'm not saying that you shouldn't surprise people. But if that situation does occur where you're buying presents for a surprise birthday party or something, you don't have to lie. There are ways to hide the truth without lying. If somebody calls you when you're out you could say something like "It's a secret" or "You'll find out later". I personally would make a joke about it and say something ludicrous like "Sorry can't talk right now it's hard to fight off these sea monsters with only one hand!" and then perhaps mimic the sound of being attacked by a whale. Namely whale noises and splashy sounds.
The biggest thing is that you don't have to lie to people to get what you want. I prefer being very open in my relationships with other people and telling them how I'm really feeling. It does no good to hide the truth when the other person can help you, even if that help simply means leaving you alone for a while.
So, if you're going to cancel some plans, be honest about it. Or at least make some whale noises.