If you haven't get one yet, I highly recommend you to get a chain necklace or bracelet. Many options are in the stores; an absolute chain or designs with crystal flowers or with pearls can be combined with basic tshirts and shirts.
My choise would be the golden chain necklace at the 1st pic. Below pictured options can be found at Zara and Mango stores. You can also find different options at H&M.
Which one is your choise?