I am polyamorous and have been for about 10 years. I've never been married, so I can't speak to opening a marriage. I think you need to get very clear within yourself and with your husband about what is and is not okay for you both before you start messing around with other people.
I think casual sex is a very difficult thing to manage, except with strangers or people you don't see regularly. If you start fucking the same person on a regular basis, chemicals kick in and it is hard to keep it casual. This can be EXTREMELY destabilizing to your existing relationship.
I also think casual sex is usually not that great. Sometimes you can really hit it off with someone, but a lot of the time it really takes a while to get to know your lover well enough, and for them to get to know you well enough, to really enjoy the sex. In other words, I think casual sex is over rated.
I am realizing I want two things: casual experience and a potential addition to our relationship. This is something to chew over for quite a long time.
Does your husband want an addition to the relationship? If yes, would the new person have an equal say in the triad, or would you and your husband be primary with the other woman as a secondary lover? Does the other woman need to be lovers with both of you or is being lovers with just one of you okay? Is it okay to be sexual with the new woman one on one or do you and your husband both have to be there?