I recommend joining a community garden.

1. You'll be the only dude.

2. There will be young women there who eat well and are in shape.

3. It will be easy to demonstrate your value by growing a killer garden and doing manly type outdoor stuff.

4. Gardening is a fun, inexpensive, and outdoor hobby regardless.

Do something that is cool anyway. NOTHING worse than joining some stupid club for the chance a girl throws you some attention.

What's cool?

As somebody else said, improv, if it's a good program.

Your college alumni club in the city might be totally hopping -- bar takeovers when the game is on, tons of 23-25 year old women, etc. Also can be totally lame, but worth checking out.

Check out city clubs, the old school social clubs in century-old buildings downtown. Can be awesome if they're trying to bring in younger members. If you golf, same deal with country clubs -- not going to be a lot of women members in their 20s, but FULL of the daughters of middle-age members. If you even like the idea of sailing and are near the water, there's a yacht club near you that wants members. And as a bonus people in their 20s pay basically zero initiation when 40-something new members will pay $20k or $50k or $100k.

Also, if you're a centrist-type Democrat, there is DEFINITELY going to be a political club or young professional organization that will be full of smart, good looking women. If you're in the South, there will be a group like that for you if you're a Republican, too. Republican groups in California and the northeast are full of nasty-looking Ayn Rand type dorks.

Finding the right one is tough -- most "young x" political organizations are basically "useless nerds being patronized." Filter by coolness of venue, arrive to your first events a bit late, and if a sausage / dork-fest, leave without even putting down your name.

Dancing is gonna be something where the ratio is like 10:1 men to women with most men being gay and having a gay guy vouch for you is usually a good thing with women. Women also love a guy who can dance and they'll be fighting for you since they need a male dance partner if you're doing traditional dances like Salsa. You'll also probably head out with them after dancing and be seen with tons of women which will only make you look more attractive. Fatties also won't be joining dance classes or will make up a small minority.

Lifting, martial arts and most sports are generally male dominated activities, so don't expect to meet girls there. Hell, there are supposedly four girls enlisted in my Savat class, but in close to two months there with two classes a week I only saw men (from kids to 60-somethings-year-olds)

A few ones already mentioned that are really good are Yoga/Pilates and Dancing, as they are not only great skills to have (core strength, flexibility and dancing skills) but women flood these kinds of classes.

The main thing I'd suggest is getting into acting/theater. I know it's something overlooked a lot these days, but it's an activity that really improves your speaking/public speaking and interaction skills, as well as being flooded with women (and most men there are gay)

I'm also taking singing lessons, and there's a lot of women here too (though less). Most artistic classes like painting, drawing or even sculpturing will be either 50/50 or less in a men/women ratio. So pick a subject you're interested and go for it.

Yoga, it should compliment your BJJ. You'll also find health minded women.