Koshien Stadium is the site of the finals of two national baseball tournaments for high schoolers in Japan (referred to as "Spring Koshien" and "Summer Koshien"). Making it to Koshien means that your team is extremely successful, one of the best in the country. Making it to either of these is a dream for many Japanese high schoolers who play baseball. Players who make it to Koshien often have successful professional careers after high school. In addition the sand from Koshien is literally a sacred object in Shintoism (there is a shrine outside the stadium). Losing teams at Koshien are allowed to take a bag of the sand home as a souvenir, which is to teach them the value of earnest effort even in defeat and supposedly holds life lessons.
As such, for many Japanese high schoolers, Koshien (and anything associated to it, especially the sand) is something of a dream. Baseball is one of the most popular sports in Japan, and these tournaments are famous enough that almost every Japanese person would know them and understand this reference.
For a sports-fan like Ikumi, this seems like a perfect present. The fake Koshien sand is actually quite cheap, but it seems valuable given the history and tradition.