Re: What Type of sentence..
1. Rather than letting him take the reins, slip in some of your own sexy ideas.
"To take the reins" = "to take control of the situation".
"Slip in" means "discreetly mention", and is an imperative.
The first clause presents the course of action the addressee should not take; the second clause presents the speaker's suggestion for an alternative course of action.
Thus you might paraphrase the sentence as:
2. Instead of allowing him to take control, discreetly mention your own sexual preferences.
Not a professional ESL teacher.