What is it that you enjoy? What do you want to do? That would make a big difference in recommendations.
Chiang Mai - we loved the Thai Elephant Conservation Center in Lampang (about an hour away). The Sunday Walking Street had good shopping, good food and a great atmosphere. Doi Suthep is a stunning temple complex, but the nearby Meo village is disappointing at best. The Night Market many seem to love we found to be rather average, with most goods being well ovepriced and poor quality...atmosphere was just touristy rather than Thai like the Walking Street and sellers were rather hardened-some, rather than bargain, would turn their backs if you didn't want to pay their hugely inflated price. Umbrella village and craft centers...we weren't impressed...large factory type showrooms where a few artists sit around and start to work when tourists show up, clearly guides/drivers get a fee whenever they bring someone, and buses are welcomed more than cars. The smaller shops right in town were more enjoyable with much friendlier staff and more items that were unique and not mass produced. I'm sure you might find something in the big craft centers, but they were not our style at all. Cooking class with Yui from
www.alotofthai.com was very special...we had a private class with a customized menu...she was the only place out of 4 I contacted that was willing to customize for us, and it was excellent, far beyond our expectations.
Phuket may be rainier in July, but you never can count on the weather. There are areas of Phuket that are overrun with tourists and all that goes along with that, like Patong, which I find to be a place I don't want to even visit, never mind stay. But there are areas that are very quiet and peaceful, like Mai Khao beach where the Marriott is and north from there onto the mainland up to Khao Lak. If you want water sports and beach time, you would be better in Koh Samui, as the seas are typically rough this time of year and the likelihood of rain is greater in Phuket than Samui. If you wanted to do other things, like John Gray's trip which goes rain or shine, cooking class, spa, maybe a drive up to Khao Lak, look around historical Phuket town, temple visits...then the weather wouldn't matter. There is plenty to do in Phuket...it's a big place with really something for everyone.
Siem Reap...that's another fine idea. It won't ever get less touristed, so now is as good a time as any to visit. In addition to temples, there are also markets (good and goods), Tonle Sap, a bird sanctuary on the lake, small villages...plenty, plenty to fill 4 day easily.