Were you close or did you know each other during your college days? If yes, you can thank the said individual on a personal level, depending on how your interpersonal relations are.

If the two of you did not interact much/ at all during college, and it was only at this internship that you met and meaningfully interacted, tread very carefully.

This might be marked trollish due ti what isbstated in the next paragraph, but I'll be blunt

Giving a gift or some memorabilia sounds problematic, even if I assume both of you are gentlemen. I need not even underline that if you are a lady, the associate might find himself in an even more uncomfortable position accepting the gift.Accepting gifts or any other items from interns may be regulated by some internal policy, and the associate (even if I don't assume OP's sex) might land in an awkward spot.

Nonetheless, if you really wish to express your gratitude, the best solution is a strictly formal/ professional letter, thanking the said associate for their guidance. Try to wrap up the letter in no more than 1½ page. (This too might seem excessive, but you get the point)

At my recent internship, I got to work very closely with my college senior, who is an associate at the company. I would really like to give him something as an appreciation for the guidance and the help. What would be some office or work essentials that I can gift him (and are affordable)?

I was also thinking to make him something that he can stick on his desk, but that might be too much???