For some historical sites, I think a guide will help you know better of this place. That's not necessary for a nature reserve. If you come in April or May, you can get to Emeishan. The azalea flower is the right time in blossom. You can also go to Leshan to see the Giant Buddha. These two places are all about 2 hours from Chengdu by bus. As to other places in China, you have many choice in fact. It depends on how many days you have. I like Suzhou and Hangzhou. How about detailed request on a travel destination, or a kind of. I can provide better recommendation. If there are several places you want to go, I suggest you book a tour.