My favorite time to travel in Yunnan is March - June because the weather is dry and the skies are blue without many clouds - makes for great pictures.
You have many places you want to visit so here are some suggestions.
Erhai Lake - www.gokunming.com/[...]
I would begin with a bus ride along the lake so you can see the entire place in one day and then make a decision as to what is next. No need for hotel reservations as there are hotels and guesthouses everywhere and this is not peak travel season.
YuanYang, Honghe - www.gokunming.com/en/forums/thread/9948/unesco_sites_in_china
I have been to Yuan Yang 6 times over the last 12 years BUT I have not visited since it was added to the UNESCO list 2 years ago.
Yuan Yang is like the Red Soil in Dongchuan - it is NOT one specific place but rather a large area where you must drive from place to place to see the rice terraces or the red soil fields. This is what makes these places either a curse or a blessing. Unless you have a car, you will have to hire a driver so you must be very clear and specific as to the price and the time they will drive you around. I find the drivers are vague and make promises that they do not keep once they get you in their car. For both these places you need a car although some tuk tuks will offer to take you around - Do not take a tuk tuk.
I know many people who are in a hurry and take a long direct bus from Kunming to Yuan Yang. I prefer to stop halfway and shorten my time on a bus and stop at Jian Shui Old Town which offers a lot.
Weather - Yunnan is mountain country so weather changes quickly if you go to the mountains. You can go as far as Lijiang with the same set of clothes but if you venture north into ShanGriLa you will need a heavy coat as April is still cold.
Lijiang - You do not have to pay any fees to go into Lijiang Old Town - The Protection fee (80 RMB) is required when you try to go inside any of the scenic spots. For example, when you go to Black Dragon Pool, it is actually free admission but you must present the protection Fee receipt which will cost you 80 RMB.
I prefer to stay in hotels in Shu He rather than in Old Town (hotels are newer and cheaper) because it is much more quiet. However, you must pay a 50 RMB fee to go inside but it is well worth it.
No fees to go in Dali Old Town.