Going to Jiuzhaigou is quite long by bus from chendgu (10 hours-12Hours), but there is now an airport close to the national park (songpan) so daily flights go there and return. You can stop in Songpan for a horse trek, which is quite funny and very rustic.
Lodging near the entrance of the National park is quite expensive (this trip is very popular with Chinese tourists), and there are a lot of luxurius hotels.
At the time we were there, there were a few cheap hotels in the city(the one we found was maybe the worst of our trip, very dirty, we slept in our sleeping bag).
Then we decided o go inside the park and sleep inside. I don’t know if it is possible now. It was already forbidden one year ago and the Chinese autorities were about to destroy all the nice tibetan houses inside the park where you could sleep, near the official restaurant (zechawa village), ask the bus to be dropped there.
To visit the park, your ticket is valid two days. After that you have to pay again. As we slept inside the park, we stayed three days without paying extra days both for the day bus ticket and entrance ticket, But take care : as the park is quite big, you have to take the bus and they may control your ticket (we saw one of the rare ticket controller but we were not caught), You can also walk but you will spend the most of your time not at the best places ; It depends if you have lot of time and if you like trekking.
There are paths. There are not difficult at all. Some parts were closed (but we took them). I think they do not want people walking, but just everybody taking their buses.) Note that sometime, you have to walk on the road if you do not take the bus at all.
Best advice is to try to sleep in the park, At the entrance, everybody will tell you it is forbidden but if the tibetain houses are still there, their families will welcomme you and you could sleep and eat with them for very cheap. From there, it is a good place to walk around : two main valleys, one day for each, (al though chinese tourists do it in one full day but they rush a bit). A third day to go back to your favorites places, or if the weather was not good the firt days, or if you want to explore more remote places.
The best time of the day to enjoy the park is early in the morning and late in the afternoon, like everywhere else juste to avoid the crowd.
Any others questions welcome.