if im wondering in the streets in my city,its usually just my 18-135 lens during the day or the da35mm f2.4 (or A-50mm f1.4) at night. However, when im travelling abroad or on holiday, I usually take my Lowepro messenger bag (the large one) which fits my K-50 and a mounted lens, usually the DA 18-135mm, and have room for 4 more lenses, the samyang 8mm, the DA55-300mm, the DA35mm f2.4 and A-50mm f1.4. Usually have my camera on one side hanging from my optech sling strap, and my bag on the other side. in the bag theres also a couple of cpl's, a spare battery, blower, lenspen and a couple of microfibre cloths.
I only leave behind my sigma 24-70mm f2.8 which even though its amazing, its really heavy and I only use it for special occasions like weddings,christenings, stage photography etc.... if i plan in renting a car during my holiday, I also take my tripod with me as well.
I do like to be walking around light but sometimes for street photography you do need a few lenses, especially a long telephoto, since you can't really go up to people and starting taking pictures...or at least i cant (yet)!
however i do need to add a couple of more primes to my collection