For instance, something that easily puts an impression on others. Note this is slightly different from "impressive," though that word also fits the meaning.
Isn't this simply persuasive?
From the dictionary Google supplies:
good at persuading someone to do or believe something through reasoning or the use of temptation.
synonyms: convincing, effective, cogent, compelling, potent, forceful, eloquent, impressive, weighty, influential, sound, valid, powerful, strong, effectual, efficacious, winning, telling, plausible, credible ...
If someone is impressionable they're open to manipulation, so manipulative might be an option.
Note that unlike persuasive, as already suggested, manipulative has negative connotations. However, I think this is apt, as being impressionable isn't usually a positive trait either: it implies one is easily led astray.
If I look at someone and say they are impressive or persuasive, then in that scenario, I am the "impressee", not the "impresser".
From the point of view of the impresser would mean that the other person is easily impressed. Gullible or naive fit loosely here. Off the top of my head, I can't think of a word with a positive connotation. Even "child-like" can be bad.
If you're actually looking for a synonym for impressive, I'm sure the thesaurus could help. The first word that springs to mind when describing something that has "left an impression on me" is memorable (or unforgettable).