“I need to go. I have something to do.” What words can we use here instead of “I have some to do” if we don’t want to specify the thing that we are going to do? Thanks.

I need to go. I have stuff to do. I need to go. I have things to do.

“Things to do,” usually.

“I need to go” is usually enough on its own. We know you have things to do. You can say you have something to do, you have an appointment, you have a previous commitment, you’ve run out of time. It never has to be more specific than that.

Fuck off, I got work to do.

“Gotta see a man about a horse”

Context and intended level of politeness are very important considerations here. If I were attending some kind of function with people I wanted to show a great deal of respect or deference, I might say something as formal as "Well, I'm afraid I have to go now, I have another engagement." If I were just hanging with friends I might say something as casual as, "Gotta go, I have somewhere to be."

"I have some to do" is not a thing. That might just be a typo, but it's not an option. I have to go. I need to go. I'm going to leave. I have errands to run. I need to take care of something. \-------------- Remember - not an English lesson: You don't have to justify your decision. "I'm ready to leave. I'm gonna go," is more than sufficient. Maybe you'll take a np. Maybe you'll go to a movie. Maybe you'll just walk around the block. It doesn't matter. You're leaving and you're free to do so.

"I have to run some errands" is nonspecific though perhaps not quite so vague as "something to do"

I’ve gotta go, these dreams aren’t going to crush themselves

You could just leave off the second have. Needing to go implies that you have something else to do.

In the UK, you'd slap your hands on your knees and go 'right', then get up and leave.

'going to see a man about a dog' is the famous one

I have plans, I need to go. I have another obligation I need to get to. I have errands to run today, I'd better get to it!

In addition to what others said, you always have the option of using one of the more ridiculous sounding idioms in English: "I need to see a man about a horse". But you should probably stick to something more discreet, lol.

It would just be various synonyms. "There's something I have to do." "I have plans with [someone/something]" "I have some other things I gotta do." I find it difficult to imagine a scenario where you would want to or need to switch things up. Sort of like the people who always greet with "Sup" or "What's up".

I need to go. I’m very busy today.

There used to be an old [BritEng] expression "I have to see a man about a dog" that meant "I have something I need to do". Don't think anyone says that anymore though. More useful for your query, I guess, could be "I've got something on" which sort of implies an event or prior engagement you need to get to.

You can always throw it back onto the other person to make them feel respected (also using "it" instead of specifying or saying "things"): "I better let you go" "I'll let you get back to it now" "I'll leave you to it" "I won't keep you any longer" Followed by your usual valediction (bye, cya, later, good day)

Just say "I need to get going and catch up on some things" or something vague like that. That gets the point across.

I have an appointment.

I'm busy I've got things going on I've got errands to run I have to go do something I need to take care of something

you could say “i’m really sorry, something has come up so i have to go”

This reminded me of the classic responses: "Where are you going?" "Out." "Who will you be with?" "People." "When will you be back? "Later."

“Something” is already nonspecific.

“I need to see a man about a dog/horse.”