People, especially certain cultures, give Mary roses and also give Mary and the saints milagros, jewelry, rosaries, and light candles to them for thanksgiving as well as for prayers. At some shrines you can buy candles actually in the shape of body parts like legs and hearts and burn them in thanksgiving or prayer request concerning the particular body part.
I will usually say a heartfelt thank-you prayer to Mary or a saint who has helped me. I will occasionally light a candle or place a flower before a Mary shrine. I am not from one of the cultures that gives the milagros, jewelry or rosaries so I don’t do that. If I wish to give a gift I am more likely to make a donation to the saint’s shrine or to a Catholic charity, as it seems a more practical way of giving thanks.