LIFX Smart Lights
While there are many options for smart lighting, I feel very strongly that these guys do it best. LIFX lights come in various sizes (link will take you to several options). Talks to Alexa and Apple Home Kit.
Colors are ultra-vibrant, and no hub required!!!
WeMo Smart Plugs by Belkin
Plug anything you want into this smart plug and instantly be able to control it with Alexa or Siri via Apple Home Kit. Please note: I only suggest using this with appliances that can be left in the "on" position (i.e. lamp, fan, etc). Otherwise, this will not offer any significant benefit.
Amazon Echo (2nd Generation)
The new second generation of Amazon's Echo has better sound and a more compact size. If you want to turn your home into a Smart Home this product is a necessity (unless you want to opt for Apple's Home Pod)
Nest Smart Learning Thermostat (Gen 3)
From everyone I know who owns one of these, this one gadget can save you significantly on your heating/cooling bill. Note: This does not work if you have electric heat.