If you don't want to having to carry a bag around, then I'd recommend a high end compact camera instead such as a LX100, FZ1000, RX100 (I, II, or III), RX10, or G7 X. The large aperture lenses coupled with 1"+ sensor gives you at least as good image quality as your old 450 + 18-55 and in some cases better and with more zoom range on both ends (wide and tele).
Anything interchangeable lens camera (ILC) is going to need a big pocket or bag, so if you weren't hauling around your Canon 450D before, you probably aren't hauling around any other ILC, either even if it's a little smaller/lighter like the X-E2, Sony E, Samsung NX, Micro Four Thirds or Nikon 1.