Dragon eye (龍眼), commonly transliterated as longan or lóng yǎn in pinyin, is a tree native to Southern China. You can buy the little fruits in most supermarkets and also often from the streets in Nanning, if it's the right season. With a bit hard brown shell that you need to squeeze a bit to crack it open, the sweet fruit flesh is white, with a large black seed inside. Don't eat the shell or the seed.
There are rather few sightseeing spots in Nanning, but the nearby Qingxiu mountain park is perhaps the most noteworthy, so I recommend visiting there. On top of the Qingxiu mountain, there is a pagoda from which you can get a big picture of the city. I recommend taking an electric bus to get to the other side of the park where the pagoda is located.
If you have already visited the Qingxiu mountain park, you can get out of the Huangmaoping park at the North-West corner (trailhead T2) at Zhuxiu road and walk around 15 minutes North-East along the Zhuxi avenue to the Nanning International Convention Center, which is a pretty flower-shaped building, and the pretty lake park with some tourist attractions on the other side of the avenue. I especially recommend this location at night time, when it is nicely lit. To get there more easily, you can take bus 89 or 34 (1 RMB), or 213 (2 RMB) from the park side of the Zhuxiu avenue to get there. If you would like to visit the Nanfu park and Nanfu Lake, you can also take a 15-minute walk from the Huangmaoping park to North-West along the Jinchun Road.
I am making this cache as easy to find as possible, in case you can't or don't want to use GPS. You can find a map of the park at least in the North-West entrance of the park (waypoint T2). The map has several coordinate points in some coordinate system. By those, the cache is located at about X=2522336.000, Y=537989.000 (a rough estimate), that is, about half-way between the two neighbouring points, but a few meters south of the more southern point.
Note the around 500 meter map error in street maps in China, so street maps may show the location incorrectly! (It shows near the park entrance in the street maps, which is incorrect.) The satellite map should be correct (at least in Google Maps).
At the cache location, you can find two big longan or dragon eye trees bedded in a stone-bordered bed, one 10 meters north of the other, and several more around... There should be couple of small stone stools (two others have gone missing) on the east side of the northern tree. There is a building some tens of meters South-West of the trees.
The park is usually quite peaceful with relatively few people, but note that Chinese muggles may be especially curious about western people snooping around.
If it is the right season, you can find eyes there watching over the park, which I find dear.