Good suggestions above. If what you want is more about being fast paced and lots of variety, ER would be a good choice and offers the benefits of shift work and no pager at home (this is a big deal). However, be ready to work some night shifts and holidays.
If you truly love being in the OR and doing surgery - which is a whole other thing compared to just doing procedures in the ED or office - then look into some of the more lifestyle friendly surgical specialities. I'm in ENT and love it. Anything that keeps you out of a GenSurg residency is a good thing, because from what I've seen and experienced GenSurg is pretty miserable. Urology can be a good option and they need more women. Ortho is good too. The problem with OB is that you are stuck with being called in for deliveries at the end of it all unless you do Gyn only or Gyn/Onc, but Infertility is a cush subspecialty that you can enter from OB/Gyn, one of my old med school classmates is doing it and loves it.
Otherwise, if you really want the best lifestyle possible do Derm, Radiology, Anesthesiology, Path or Rad Onc. You can do a fellowship after Derm too that lets you do more surgery like Mohs, etc. Of course, all of these are super competitive to enter so be sure to do well in your 3rd yr clerkships and get some research under your belt. Anesthesia would be a good fit for you except for the whole sitting there during the case charting and waiting, all the action is at the beginning and end of the case, so you might get bored.
Internal medicine is not the place to look for a great lifestyle either unless you stick with only outpatient work or go the hospitalist shift work route. IM residency looks pretty exhausting to me and the payoff at the end isn't that great unless you do a fellowship.
Another thing to consider is that some surgical residencies incorporate a year or two of research. While this lengthens the process it is a VERY popular time to have a baby.
While it's possible (probable?) that about 1/3 of the places I interviewed were just trotting out all their mom-residents/pregnant residents, I think that at least at some programs, things are changing. I also think that the sort of place that was willing to interview a non-traditional student (just seeing my BA in 1999 in a non-science field blares out non-traditional) who didn't have stellar Step 1, but then pulled it together on Step 2, is also the sort of place that is okay with women having families. At my home program, our PD (who's not known for sugar-coating) actually recommended during the interview to not view residency as a step on a pathway, but rather as a destination, and that people should buy homes and have children during residency. We do have a fair number of mom-residents here, and our surgery residents go over hours far less than our OB residents. With that being said, I think there are very few residencies, surgical or not, that would be possible to do with children if your spouse doesn't have flexible hours or you have childcare available 24/7.
An off the beaten path recommendation - how about Urology? Sounds crazy for a woman, but there are woefully few female Urologists, and lots of women with bladder/pelvic floor issues looking for help. The residency is a little less stressful than gen surg (at least at my institution) with regards to call, etc. Your internship year would still be bad, but the remaining years easier to handle. Plus, in the end you would likely be able to write your ticket just about anywhere give the lack of female docs in the field. Along with that comes the ability to decide your hours, etc.
You may want to invest in Iserson's Getting into Residency (it was a gret resource throughout my med school years, not just during third and fourth year) - they have some great sections on helping you figure out where your skills and intersts would be best served, and actually opened my eyes to a lot of specialities that I never would have though of otherwise!
Another option (and I have to throw this out there, especially since you mentioned your multi-tasking and need for intellectual challenge) would be hospitalist medicine (through IM or FP). I also craved an intellectual challenge/diversity of cases and fast-paced lifestyle, and have found that my job as a hospitalist is very compatible with my life as a mom. I seem to do far fewer overnight shifts than my EM colleagues, too, which is a huge added bonus!