How is it being a 12B Combat Engineer on active duty?

Lewis, Irwin, Carson, Riley, Campbell, Bragg, Bliss, Hood, Drum, Stewart, Polk, Benning, Leonard Wood, Alaska, Hawaii, Germany, Korea, Italy... as an engineer you can pretty much go anywhere.
If you end up in a Bradley unit, you will live in the motor pool or be in the field OFTEN especially if you get stuck at Riley. If you Get to a light unit, you will get more tactical knowledge. Try to get yourself to a Stryker unit. In my opinion, That’s where it’s at, you get the best of both worlds. Also if you don’t know demo, I’d advise opening a book and figuring it out. Be pretty embarasing to show up to a unit as an nco and your Soldiers know more about your job then you do.
Good luck!

I can't speak of being a 12B specifically as I was an 11B in the regular army as well as in the guard (with a few years in the middle as a 12B). BUT, I can say that in some ways they are the same and in some they are very, very different.
One thing I noticed, and this is very unit specific.... the 82nd or 101st is going to be a bit stricter than a unit at Ft. Lewis (from what I've been told it was a culture shock for some people). But it is a little stricter overall. Almost nobody on a first name basis. A little more separation between ranks. A little more petty stuff at times.
But one thing I found in the Guard a lot of times was trying to cram a full months worth of stupidity into a weekend drill.
A rifle qual that took all day, or all weekend for just a company in the guard would have been enough to cycle a complete infantry batallion (including the anti armor company in the 101st) through... with a day left over.
Actually standing at parade rest for NCOs senior to you.
A lot of time hiding out playing video games.
A lot of time cutting the grass, or picking up trash on the side of the road, or acting at gate guards, or other menial stuff.
More time for actual training. When I was a guard 12B we had just a couple days a year, usually just at AT when we even saw live demo. If you were screwed and had KP that day... well you were just screwed. I volunteered a couple times so a new private (or someone who reclassed and maybe hadn't been through MOSQ, or didn't have a lot of demo time or was still nervous about it) could have the experience. I'd handled live demo more as a 11B in the active army than I did in the Guard as a 12B until were were doing some trainup before deploying.
There is a good chance you will be treated more like an adult. Have you deployed? If so, remember the BS pre-mob training? Where you were locked down for a couple months, couldn't go off post without commander's permission, couldn't even go to the damn PX a lot of the time? Different units had different rules of course, but what we did for a couple months, regular army units here did for just the week before getting on the plane.

Best of luck to you. I was in the Iowa National Guard as a 12B, but Commissioning was the only option available that allowed me to transfer to Active Duty.
As far as Duty Stations, you can choose pretty much any installation.
Ft. Drum, NY; Ft. Bragg; Ft. Campbell, KY; Ft. Leonard Wood, MO; Ft. Riley, KS; Ft. Carson, CO; JBLM, WA; and many more. The only limiting factor is manning requirements and what you want.
My first duty station was Joint Base Lewis-McChord (JBLM), WA with the 1st Stryker Brigade. It was a fantastic location and the unit was great.
Active Duty life can be pretty busy, depending on the Unit. In the last 4 years my unit conducted 3x NTC rotations and a 6 month Pacific Theater Rotation (Thailand, South Korea, and Philippines.
Overall, I'm happy with my transition to Active Duty and will be glad to answer any other questions you have.