Here's a list of everything that you can use that will stack:
- Milk tea: 8%
- Exp elixers: 10% / 15%/ 20%
- Costume: 10%
- Helmet exp gems: 10% x 2
- Book of combat or loyalty exp scroll: 50%
- Guild exp buffs: 10%
- Value pack: 30%
- Pet exp traits: 3% / 5% / 7% x ?
- Daily grinding hour buff: 100% / 200%
- Villa exp buff: 10%
- Cold dark beer (event item only): 20%
- Bandit town quest buff: 30%
- Night time buff: 50%?
- Weekend exp buff or Olvia channel buff (not implemented yet?): 200%
- Golden Bell: 100%
- Knowledge progress reward: 3% - 7%?
Let me know in comments if I forgot to add something. :)