Nanning isn't really a touristy place. I've been here 2 years and I can  hardly think of any 'must see' places. Perhaps Green mountain, Nanhu park / lake and a quick drive down Minzu Dadao will get you to the central 'square', but that's about it.

What I would recommend is getting your fill of fresh fruit, as that is hugely abundant, although, this time of year, the more 'exotic' fruits are out of season, so you are down to oranges (the skin is coloured green though), Longan, (still in season....just), bananas, Durian, and a few thousand others. 

Google Nanning tourist attractions and read thru to find any you would like to see.

There are many if you know where to look for them, and depending on your likes and dislikes.