I want to start a proect on Arduino, one that is interesting/complicated enough to keep me challenged and intrugued, easy enough for me to actually learn something
What's your skill level?
and useful enough for my dad to stop saying that I am wasting my time.
If you are learning something, you're NOT wasting your time. Programming is NOT EASY and not everyone can do it (although it's easy to get started with the Arduino) so you are exercising your brain! There are a lot WORSE THINGS you could be doing with your time!
My dad wants me to do a project that will actually help make a job easier,
I suppose that makes sense. But, sometimes it's OK to build something that's just fun! It's also a good idea to build something "permanent" that you can keep, rather than breadboarding something and then tearing it apart when you''re done.
If you have any other hobbies (besides electronics & programming) maybe you can make something for you hobby.
The trouble with making a "useful tool" or a "useful item" is if that tool already exists, it's usually cheaper to buy it than to make it yourself. It's more fun if you can think of something you want, but that you can't buy. (And, then it has to be possible and practical to build it.)
All of my Arduino projects have been sound activated lighting effects. I've made 3 of them and in many ways they are similar. One is a "giant VU Meter" (24 LEDs in a row, spaced a few inches apart so it's 8-feet tall, with one on the left and another on the right). Another one is a 4-Channel effect that runs 4, 8, or 16 floodlights, depending on what's plugged-in.
You make a very simple lighting effect, or a complicated one. The simplest thing you can do is make a light (or LED) come-on when the volume is above a threshold and turn-off when it's below the threshold. Or, you can set the threshold to the moving-average so it automatically adjusts to volume control changes or quiet or loud songs. (My lighting effects use a 20-second moving average.)
One step beyond that is to set-up a number of lights with a random pattern, and toggle the lights to the opposite state when the volume exceeds the threshold. I call that a "flicker effect". (That's one of the "modes" for my 4-channel effect, and my "VU-Meter" can do that too... It gets boring with one single effect so my program changes modes about once per minute, and I have 7 modes with variations of each mode.)
Many years ago, with a different microcontroller, I made a "wake-up light" that fades-up the light over my bed in the morning, and after it gets to full brightness it makes a gentle "beep". After 30 seconds I get another "beep". Then it starts counting the minutes, so at 2-minutes (and again at 2:30) I hear "beep-beep", then after 3 minutes "beep-beep-beep".
In the past with a different microcontroller, I also built a car alarm.
P.S.
My 1st Arduino project (but not my 1st sound-activated lighting effect) was a HUGE waste of time! But, it was fun for me! My van had "rope lights" embedded in the ceiling that just had an on-off switch. When my sister and her kids were riding with me, she told them that I had put those ceiling lights in the van. I said, "No.... If I'd put them in, they'd flash to the music!" After thinking about it for a few months, and after a couple more months of designing, programming, and wiring, I had the lights blinking/sequencing to the music. I can't really see them while driving, and other than showing my sister for a few minutes, nobody is going really appreciate it but me!
