I'm assuming 40g means 40 breeder, and not a tank that has the same footprint as a 55, but is shorter.
If this is the case, there are hundreds of species you could keep in there. Almost any species of North or Central American rat, king, or milk snake would fit in it perfectly fine. I'm a huge fan of North American rat snakes specifically- any "obsoleta" subspecies. Black rats, texas rats, yellow and Everglades rats are some awesome animals. They're fairly active as snakes go (much more active than some other species), and can be really impressive as adults. Wild caughts, and Texas rats (even CB) have a reputation for being nippy, but most will calm down with some handling. Every captive bred I've ever had (except for a couple Texas rats- these seem to stay a little bit wilder, although I've also seen tons of calm ones) were perfectly fine with handling by the time they were 16-24", which is about the size where you start to even feel bites from them. Smaller than that, and their teeth really don't break skin, or they barely do if that. Look into some of the morphs out there, and you can find some really cool things. Lemon, rusty, two different strains of albino, leucistic, whitesided, and brindle are all available in black rats. In yellows and/or Everglades (depending on who you ask, and the bloodlines involved) are also available in hypo, and whitesided or "ghost" morphs. There are others out there, too, but these are the most common.
Honduran milks are awesome- as was already pointed out. Tons of color options, and they're fairly heavy bodied, slow moving snakes as adults. My favorites are tangerine dream, and You could also look into Sinaloan milks, or really, any other species of milk snake. Mexican black kingsnakes are also really impressive as adults.
There are a bunch of varieties of California kingsnakes, as well, and they're generally regarded as one of the best first snakes- super calm, and really easy to take care of.
Rosy boa was something mentioned as well- rosies are awesome! I have a mid-baja rosy that is a really nice snake, if a bit lethargic at times.
There are several dwarf or insular species of boa constrictor that would work as well. Male Hog Island boas can live in a 40, as can Corn Island and Pearl Island boas. These are getting up there in price, but they're BEAUTIFUL animals, and have that slower metabolism and generally calm demeanor of larger boa constrictors.
You could also look into the Pituophis genus- pine, bull, and gopher snakes. There are several color mutations of these, as well. They tend to be a bit higher-strung than the other snakes that have been suggested, but as Fishfreak said, they calm down with handling.
I've never kept Bairds, but they're beautiful, handleable, and would go well.
As someone else pointed out, most carpet pythons get too large, but you could look into Irian Jayas, and even male jungle carpets would live in a 40 breeder, especially if you look into bloodlines and find one of the smaller bloodlines. People tend to over-exaggerate size for jungle carpets at least. I've kept them for years, and know tons of breeders, and most everyone agrees that while 8-10 feet is possible, most bloodlines produce males 5-6 feet, and females 6-8 feet, unless they have some Coastal blood mixed in. Carpets are extremely variable, and dependent on bloodline- small, bright colored adults produce small, bright colored offspring. I've only ever had two adult male jungle carpets, but both of them were bought as adults and were under 6'- perfectly fine for a 40 breeder.
Keep in mind that snakes of the genus Lampropeltis (kings and milks) are cannibalistic, and should be kept singly. If the missus wanted something smaller like rosy boas, or even some of the rat snakes (corns or Emory rats, specifically, although really most North American rats), she might be able to keep a pair in a 40, which adds some more excitement- the possibility of keeping two different color morphs, and even breeding the animals. Raising baby snakes isn't very profitable, but it can sure be fun!