An Out Of the Box option: Pontiac Solstice GXP
Nice-looking, decent handling, cheap to maintain, and 300lb-ft of torque at 1500rpm with a factory tweak of the turbo that pretty much all of them have had by now; one of the better factory stereo systems I've encountered. Downside is the interior quality isn't up there with others (pleather seats, hard plastic dash) and trunk space is almost laughable. I've owned 4 Porsches, 5 BMWs, and a Rolls-Royce, and the car that the most women have complimented me on was my Solstice, so I'm guessing the Mrs will like it. The designer that did the Solstice left to do the first Tesla Roadster and is still their head of design. It's weird to me how people buy Miatas and add turbos or swap engines when the Solstice GXP is already there, a roadster with real muscle.
My take on some others mentioned:
The Honda S2000 is adored by younger types, and the reason insurance is so high is that they're frequently stolen and stripped for parts to go into other Hondas; the seats are
hugely desired. Nice car if you don't mind keeping the revs up.
The Z3 has, imo, a poor chassis, though average for the times. The E85 Z4 was a big upgrade - more than twice as stiff - and is as reliable as the E46 3-series it's based on. That means not many engine issues, but replacing the water-pump every 60k miles. I suggest the face-lifted 3.0si. The performance of the 987 Boxster without the maintenance worries.
The 987 Boxster: the performance of a BMW Z4 with slightly better handling, much better brakes, nicer interior (the switches look worn quickly, though), an extra trunk and more leg room. The specter of IMS looms but I've always felt that was over-stated and it didn't worry me when I had mine.
It's so close on these two that if I were sports car shopping, I'd pick the Boxster if I wanted a roadster and a Z4 coupe if I wanted a hard-top.