Go with some hybrids. They are a mix between mtbs and road bikes and would be perfect for the limited use you describe.
You don't need a road bike for around town riding(trails and sidewalks/neighborhoods) because you are stopping frequently and you would not be in the drops(the low part of the handlebar). You also don't need a mountain bike as it would be unnecessarily heavy.
- Look for bikes with no suspension. Suspension = heavy. Suspension = moving parts that don't work well if not assembled properly and a hassle to maintain if needed.
- Look for bikes with 28mm - 38mm with 32ish being a pretty good compromise. that is 1.25" wide and will be good to absorb the typical bumps of town riding while not being extremely heavy and thick. The wider you go, the more rolling resistance, typically. Speaking of- you don't need knobby mtb tires like your old bike. That will only make it more difficult to pedal.
- Look for bikes with a flat bar variation. These are great for shorter rides and town riding as you are more upright which is more comfortable on short rides and helps in town to be seen by cars and to see up ahead.
- Go to a few bike shops and ask a ton of questions. See what they have and price it compared to other shops. If the costs are too high, you can always go the Craigslist route since you will now have an idea of what type of bike and sizes are needed. Take the bikes to whichever bike shop you visited for tune ups. Itll ensure the bikes are properly tuned and in good working order, and will toss some money the shop's way since you hopefully learned a lot from them in the process.
- $300 per for used should get you something workable. $500-600 new should get you something workable. Ballpark figures, of course.
No idea what to say about your girlfriend, only that its a shame to hear she is hesitant to ride because of what happened when she was smaller. Best idea I have is to go to a dedicated paved bike trail the first few times you guys ride. Drive there, then start and end on the trail. There will be no traffic and little need to brake hard or often. Hopefully that eases her back into riding and she can get a lot of confidence.
Good luck