I agree on upgrading the lens to something faster (ie f/2.8). The question I always ask when I think about upgrading or adding new gear is, "what am I missing?" or "what do I need the new equipment to do?" That helps me to identify a short list, it also helps the good people on here know which direction to pint you in.
After looking at what you've said you like to shoot, be prepared for people to suggest the Canon 6D. That jumped straight into my mind, it is very good for low light as you get usable pictures at high ISO. The downside will be that because it is FF, you will need to replace the 18-55 (or 17-55 if you get one) and f2.8 lenses for FF don't come cheap.