I'm from the St. Louis area and lived for several years in Chesterfield. St. Louis is a great place--big small town feel, lots of different neighborhoods, great traditions, great baseball, "Rome of the West"...Feel free to PM me about it.
Incarnate Word is the other parish (besides Ascension) in Chesterfield. It is located at 141 & Olive Blvd, a couple of miles north of Hwy 40 (and a bit east of Logan). Ascension is farther west. IW has one of the most active Life Teen programs in the Archdiocese, and I believe they have a fairly active young adult ministry along with that, though I'm not entirely sure. I think that most of the 20-somethings are involved in running the teen ministry, rather than in 20-something only events, if that's what you're looking for. Both IW and Ascension cater to families of school-aged children, as both have large elementary schools and most of the parishioners are families with small children. IW also has adult sports leagues and singles events (though that doesn't seem to apply!), adult bible study, and a men's group, along with a perpetual Eucharistic adoration chapel. It's a strong parish, and I belonged there when I was growing up.
Others have mentioned St. Joseph's in Cottleville, which is in St. Charles County (FAR away from Logan...20-30 mins), and St. Gianna (which is brand new and in Lake St. Louis, I think...also probably 20 minutes from Logan). Another large and active parish relatively near Chesterfield would be Sacred Heart in Valley Park.
I'm not sure what your girlfriend/fiancee plans on doing, but Chesterfield is pretty far out in the western suburbs of St. Louis. It's a great area, but the cost of living there is certainly higher than in other parts of the county, and it is more common for families with children to live there than young adults. Students and young professionals tend to live closer to the city, often in trendy areas like the Central West End and U-City/Delmar, or in some of the new loft districts downtown. I say this not to necessarily endorse those as places to live (they definitely have higher rental rates than most places in Chesterfield), but to mention that some of the most active young adult ministries are down in that section of the city (at the Cathedral parish and at St. Francis Xavier, which is adjacent to SLU's campus). I just don't think that there are as many opportunities for young adults (20s and 30s) in parishes out in Chesterfield, as there just aren't that many of them living out there.
St. Louis is a great Archdiocese, and there are a couple of archdiocesan-wide young adult groups and plenty of volunteer opportunities. Check out archstl.org or the archdiocesan newspaper, the St. Louis Review. You can search for parishes within a geographic radius on the archdiocesan website. St. Louis is definitely a place where there are ample opportunities to nurture one's faith.