Use MS Word synonyms feature. Just type a word, right click and select synonyms. It gives words like scholarly, intellect, cerebral among others. Question is, how long can you keep this up? Only trust people who can see that there is something special about you other than big words.
Then simply telling you would defeat your purpose. If striving to impress someone, you need to not only know words, but also their meanings, and how and when to use them. Read all you can, look up any words you don't know(the web is great for this).
acumen, agility, alertness, aptitude, brainpower, brains, brightness, brilliance, capacity, cleverness, comprehension, coruscation, discernment, intellect, judgement, luminosity, mentality, perception, perspicacity, precocity, quotient, sagacity, savvy, trenchancy... are a few synonyms for intelligent. These words can be used in a variety of ways, a suggestion would be to look them up in the dictionary to verify their usage.
For large spelling words you can use words like "intellectual," "knowledgeable," or "well-informed. " On the other hand, these are adequate: "educated," "experienced," and "scholarly. "
The word intelligent has various connotations; as such, you must discern the context in which the word will be used. If describing someone as very observant and perceptive, try using the word "perspicacious. " If they seem to know everything, use the word "omniscient. " If they are simply judicious and rational, use the word "sagacious. "
There are several "intelligent sounding words" that mean that you are smart, or at the least imply competence in an academic area. In fact, "academically successful" is one good way to say you are intelligent. "I use elevated diction" is another way to specifically say you have a good vocabulary. A thesaurus is really your best friend in this.
I like the words "penetrating," "brilliant," and "perspicacious." You could also try the words "knowledgeable" or "comprehending" as synonyms for intelligent.
To describe a person as intelligent, you can also use words like ingenious, which means cleverly skillful, or adroit, which also means skillful, or intellectual, which means having superior intellect.
