I was able to pack everything I would need for 3 weeks in one carry on sized backpack and I also have a small day bag with me to hold my valuables. My small day bag is just a plain black zipper bag that has a wire running through it so if someone tries to cut it off and run, they can't, it also has a clasp to hold the zipper so people can't pick pocket me, it has been very valuable on this trip!
So for clothing (this does not include what I'm wearing):
1 sweater (which generally stays out of my bag just in case)
4 tops (mix of short and long sleeve)
4 pairs of pants (2 comfy and 2 jeans)
6 pairs of socks
4 hair elastics
6 bobby pins/ clips
Makeup (keep it simple):
Lip balm (an absolute must you will need it!)
I brought a tablet but never used it because I could do everything on my phone
2 pillow cases (for hostels)
Medication/gravol/ immodium ect
Cell phone and chargers
Clothing detergent for the sink
If you think of anything else to add to this list please add it to the comments!