It's really hard to find a decent bike for under $400 and it's hard to find a decent bike on Amazon (most are low-end crap).

That said the Diamondback Insight 1 is a reasonable quality bike that would probably good enough for your use case: http://www.amazon.com/Diamondback-Bicycles-Insight-Complete-Performance/dp/B00NCUFGM2/ref=sr_1_3?ie=UTF8&qid=1435611724&sr=8-3&keywords=diamondback+insight

I'd still pay a bike shop to look it over and make sure it's assembled correctly. I would check out the Giant Escape 3 or Trek 7.0FX at your local bike shops ($340 and 390 respectively).