BlazinXtreme
I'm open to all suggestions. I don't have to go diesel, but I would like something over 30mpg.
Top suggestion is nowhere near that, but it should save you quite a bit originally. My favorite used car under $15000 - overall, not just GM - is the
1997-2004 Buick Regal GS, which should list anywhere from $6000 for a decent '97 to $16000 for a nice '04. Fuel economy was an EPA rated 18/27, which probably means about 23mpg overall given the amount of driving you do, but the main benefit is initial cost - you can probably go Blazer-for-Regal if you sell the Blazer privately, and given you were prepared to spend $20000 you'd save a bundle.
The
1999-2002 Saab 9-3 is a nice car too - even in automatic, base engine guise. The base model (185hp 2.0L turbo four) was the only one available with an automatic, rated at 19/25 or about 21mpg given your driving. Early ones are going for $7000 these days; $12000 can buy you a very nice 2001. I wouldn't recommend the WRX - you'd need to get a manual to get the same fuel economy as the above cars, and since you prefer automatic you'd get worse mileage and similar performance to the two vehicles listed above. Not to mention even cheap '02 WRXs go for $16000, which isn't much of a deal since you're not saving that much fuel.
Top new-car rec is the
Suzuki Aerio. That's a GM vehicle for those keeping score at home - barely. Standard features are great and made even better with the '05 introduction of front side airbags. Horsepower's 155 but 27mpg is easily achievable. List on the base model is just $13000, climbing to a base of $14700 on the LX, with a standard 6-disc CD changer and heated mirrors. I'd really consider this one new if acceleration means anything to you (it's very on-par with the Jetta TDI except it would leave one in the dust - if fuel economy's more important, get Jetta; if performance is, go Suzuki).
I also like the Kia Spectra EX and Hyundai Elantra GLS, though they aren't GM vehicles. Truth be told, 30mpg is tough to achieve over at GM, and even tougher if you want fun-to-drive. Hell, 30mpg anywhere is tough to achieve if you want 'fun-to-drive', especially if acceleration has anything to do with 'fun-to-drive.'