friendly or kind :)
Pleasant, agreeable.
You can use kind, friendly, or a word that describes exactly how they are nice, such as generous, forgiving, or whatever word(s) apply.
good, excellent, pleasant, lovely, beautiful, agreeable, nice-looking, amiable,
Another word for nice or kind to sound more professional in a thank you note could be polite or pleasant.
great
That girl is nice. that was nice of you
rocks are very nice things
Instead of using the word challenging you can use the word gifted, special, special gifted.
Yes but if you are in school and you have to do this for homework... (possibly) then they might want you to use a better word instead of something that is used as a describing word everyday. For example, just use "Amazing" or "Gorgeous" I usually don't hear those words everyday, I only hear "Nice" so yes "Nice" is a describing word but maybe you could go with something else more creative, like a word that is not often used. But it's your choice!
Use a nice vinegertte. It will taste AMAZING.
or
Will, when, why, it, can