Zaki - Yes, Simhenge is great for collectors. I remember I had a (very) destitute Sim who was living as a Mooch and a Klepto. For some reason I sent her up to Simhenge and every butterfly was there, even the Rainbow one that has a rarity rating of 'Extraordinarily Unusual'. She caught every one, sold them and got a tidy sum.
Not sure if this is common knowledge or has been said already, but if you want to complete the LTW that involves adopting a unicorn, there's an easy way to find them. They don't appear every night, but if you go to Map View you can see where they are. It's like a dark grey smoke cloud and faint sparkles - if you zoom in, the unicorn will be underneath the cloud. In SV at least, they tend to hang out around the Science Facility.