Anthony Santander
Santander is arguably the best pure bat the Mariners could still acquire (outside of maybe Bellinger, but they seem to be out of his price range). In the last 2 years, Santander is 30th in slugging percentage and 15th in home runs, thats right there with guys like Bo Bichette, Juan Soto, and Julio Rodriguez, yet we never hear much about him.
Santander is almost a sure-fire guy to post a 120 wRC+ with around 30 home runs (though Statcast predicted he would've hit 39 home runs in Seattle last year). Adding his switch-hitting bat to the middle of the lineup would start to really make the Mariners a threat for the AL West title. Santander isn't a great defender, having him and Canzone in the corner spots doesn't give you a great defensive outfield, but you are getting him for the offense (though he can play some first base).
Santander is going to be the most expensive player on this list, both in terms of financial cost (projected $14.1 million in arbitration via MLB Traderumors), as well as the acquisitional cost, though it shouldn't be too painful considering he is a one-year rental. The trade package could be similar to that of Teoscar Hernandez's (who's arbitration numbers are ironically the same) deal last year when the Mariners sent Erik Swanson and Adam Macko to the Blue Jays for the right fielder. Both are offensive minded right fielders on a relatively high arbitration number, to a team that needs or needed pitching. I am not sure if the Orioles would be interested, but a deal like Justin Topa and Emerson Hancock might pique their interest; or more bulk, high upside could be something like Justin Topa, Walter Ford/Michael Morales, and another top 15 prospect.