At a Pidyon HaBen I have observed that the Minhag is to put sugar cubes and garlic cloves on the tray with the baby. What is the source of this Minhag? And why specifically sugar and garlic?
According to this it's because garlic and sugar are spices which last along time and get put in multiple dishes. This way, people can take some of the garlic and sugar home and thereby be able to partake in the mitzvah-food for a long time to come. A similar idea can be found here.
According to this it's because sugar shows that mitzvot are sweet and garlic is a symbol for fertility.
People who attend a pidyon haben are counted as if they made 84 fasts, tanesim. Anyone who partakes of the feast of the pidyon, is likewise credited. Since garlic and sugar leave their taste in minute quantities, we cook and then distribute food with this garlic and sugar to be mezakeh many, many, other people as if they too fasted 84 fasts.
Rav Moshe Heinemann said in a shiur that there was a time when garlic was very expensive, and was one of the spices Christopher Columbus was searching for on his path to India. Garlic wasn’t naturally grown in the European countries in that period, so they needed to import it from other countries for a lot of money. The same thing was true about sugar that it was very expensive at times since it didn’t grow in Europe. Only the rich people were able to afford these luxuries and would use it the same way people use jewelry. Therefore, people wanted to show their chashivus hamitzvah by putting these expensive foods on the tray with the baby. Nonetheless, Rav Heinemann said it isn’t an old מנהג, though if someone wants to do it then it's fine.
Rav Yirmiyahu Kaganoff suggests
Some people place pieces of garlic, sugar cubes, or candies alongside the bechor when he is brought in for the pidyon. The sugar cubes show that the mitzvos are sweet, and garlic is a symbol of and segulah for fertility. Some say that when participants take home the sugar and the garlic and use them for cooking their own meals at home, they increase the numbers of people who “participated” in the pidyon haben meal, all of whom will be blessed by this.