get in shape, become prettier etc. I'm working on it, but I wonder if that's all there is.
Woah woah woah! Danger! You're severely underestimating what it takes to get in shape and become prettier. It takes a lot. You're not there yet, not by a long shot, probably, so maybe just do it for a while and wait a bit before declaring athletic physique and attractive looks "not enough". A little modesty and realism, please! Or maybe you're already a beauty queen top model who still can't get laid. Unlikely but hey, everything is possible.
But I wonder if maybe becoming a more interesting person (the advice given for men -- not only attractive, as mandates the red pill for women philosophy) would work on my husband too.
Listen, this is just bargaining and denial that you've engaged in right there. Stuff like that shows you're trying to kid yourself because you can't handle reality. And what is reality? Reality is this - it takes two to tango. You can become queen of the moon for all I care, as long as the other person does nothing, it's all pointless. You may wish to put more and more effort into this - if the other party does not wish to reciprocate - all your effort is in vain. Remember that when making such efforts - maybe it will put things in perspective. If you really have to move mountains to get a begrudging sigh in return, you are clearly with the wrong person. It's just stupid. There are plenty of people who will require no such efforts on your part. In a world of 7 billion, you are literally wasting your life on purpose.
Whether the other person changes or does not change is outside of your control. You received one body - your own - and you have 100% control over it. That's the deal. You don't get to exert control over other people too. You have your own life to rule, and your husband has his. He can do whatever he likes, drink beer, nap on the couch all day, whatevs. He gets to decide what he does with his time, you get to decide yours.
So instead of wasting your time trying to force him to spend his time according to what YOU want (like having sex with you), maybe you should respect his right to do whatever he likes. And then also start respecting your own right to do whatever you like and spend your own time however you like.
Forget about him. He will make his own choices. You have to make yours. If you want to become a more interesting person, go for it. But don't be surprised if that too is "not enough". As long as you keep doing everything as a bargain, in order to solicit a reaction or behaviour, as long as you keep doing stuff with the hope that he will do X for you, you'll be forever at the mercy of X.
Just forget about him. Do it for you. Become a more interesting person because it makes you feel better and more fulfilled, happier, a better person. And if he's idiot enough not to see it - just leave him, there are others who will be able to see it. But you can't be doing it for the other person. If you cannot see the value in your own person, and enrich and improve yourself for your own pleasure and benefit, then any kind of "improvement" for the sake of another person will inevitably poison you with bitterness. Why? Because you lied that you wanted to become a more interesting person, and all you really wanted is for your husband to be interested in you, which is an entirely different thing.
So if you want to become more interesting, find a better reason to do it than just "maybe my husband will like me then". And if you cannot find it in you to make a change like that for you, and not for him, then it's simply not something that you really want. You're just trying to "trade" it for something else.
It's pretty simple, actually. People who do good stuff for their own selves, and focus on their own well-being and happiness become (gasp!) happy and satisfied. That's attractive to others because it looks like you've figured life out. And in a way you have - you've realized that no one is coming to save you, nobody is responsible for your happiness, but you have the choice and option to do nothing OR do something about it yourself. Who knows, maybe one day you will become kind to yourself enough to give yourself the gift of becoming open again to receiving satisfaction from a better partner. Right now you're stubbornly insisting on receiving all your satisfaction from one particular person who is incapable and/or unwilling.