What can be included in between a separable verb?
Here is what AllSetLearning has to say about separable verbs:


  ...until you understand which verbs are separable verbs and how they
  work, you'll forever be making mistakes with these verbs, even in very
  basic sentences.


BTW I think it is a quite decent summary.

Besides 了, 着, 过, and measure words, you can put other things in between if the verb is not "rigid". 

Some separable verbs are more rigid than others. For example: 发愁, 散步, 聊天, 分手, etc. Notice that the meaning only emerges when the two characters combined together. If you separate them as verb + object it just look weird.

Less rigid cases include 睡觉, 吃饭, 撒谎, etc. In these cases the verb and noun can be separately used with no problem, so you can say 我睡了一个美美的大觉, or 他撒了一个神仙来了都遮不住的谎.