Don't forget to actually think about it and give the response. Make a note of it and show your child that you did create a reminder.
Later, after you've thought of it, remind your child by showing them that note and let them know the outcome.
This way, next time you say "let me think about it" your child will believe that a resolution will actually come later. Showing your child the note will reinforce that you will be thinking about this.
Another point about answering their request at a later time. It also gives your child a cooling off period. They may want something in the heat of the moment. Immediately replying "no" while they're heated about something could unnecessarily upset them. By the time you reply, they could have lost interest.
I was thinking about it in terms of stalling for the 15 seconds it takes to make a response, instead of a reaction. But yeah, I totally agree that for those longer considerations, demonstrating the integrity to follow through is really important. My oldest is just 2, so this doesn't come up much yet. She has very little concept of tomorrow.
But I know from my experience with high school kids and with managing adults that people sort the peers and authority figures in their life into the buckets of "dependable", "needs follow-up", and "chaotic". It's like a credit rating. Maintaining that trust keeps a whole lot of other relationship dynamics healthier.
I'm in the same boat as you. My son just turned two a couple of weeks ago and I'm trying to teach him about the days of the week.
I'm using a weekly calendar to explain what we're doing on different days of the week.
Each morning, I have him mark the current day. We talk about what we're doing today, what we did yesterday, what we're doing tomorrow and the rest of the week.
After a few months of doing it, he can point to a day of the week and mention the activity we'll be doing.
Don't forget to actually think about it and give the response. Make a note of it and show your child that you did create a reminder.
Later, after you've thought of it, remind your child by showing them that note and let them know the outcome.
This way, next time you say "let me think about it" your child will believe that a resolution will actually come later. Showing your child the note will reinforce that you will be thinking about this.
Another point about answering their request at a later time. It also gives your child a cooling off period. They may want something in the heat of the moment. Immediately replying "no" while they're heated about something could unnecessarily upset them. By the time you reply, they could have lost interest.