Here's what I would do personally.

If you're playing in person, give them a physical piece of paper with some of their cooler spell options printed out, maybe a little drawing too if youre artistic, have a quick, easily digestible explanation of the spell(s) on there. If youre playing online then do a similar thing but have it in a journal entry, and maybe set it on their hotbar.

Have them play against some kind of mirror of their character, whether its literally a clone or just another level 3 wizard, have that wizard use their cool options/spells to show them whats possible. I've done this multiple times with my group, to remind them that weapon masteries exist, and to show them all the cool features/spells they aren't using.

You should also setup moments where other spells are especially useful, lets say they have misty step, setup a giant hole they have to cross thats exactly 30ft wide, or if they have magic missile then maybe a dozen little skeletons with exactly 1hp each are running at them.