I have always liked the idea of exploding synergies. This works by giving you an increase in the synergy bonus increases by +1 for every 5 ranks over and above 5. So, tumble 5 ranks gives a bonus of +2 to balance and jump. When you get 10 ranks of tumble that bonus becomes +3 to balance and jump.
As for additional synergies I look at the profession skills:
Profession (Apothecary) synergy to Heal, disease and poison only, and Craft (Alchemy)
Profession (Driver) synergy to Handle Animal, hurry 1 or 2 animals hitched to a cart or wagon or other only
Profession (Fisher) synergy to Survival, in regions you can fish only
Profession (Guard) synergy to Listen, Search
Profession (Guide) synergy to Survival, to avoid getting lost and natural hazards
Profession (Herbalist) synergy to Heal, disease and poison only
Profession (Herder) synergy to Handle Animal
Profession (Hunter) synergy to Survival, when hunting only
Profession (Lumberjack) synergy to Craft (Woodworking)
Profession (Mining) synergy to Craft (Stone Masonry) when underground
Profession (Rancher) synergy to Handle Animal, hurry herds of animals only while riding
Profession (Sailor) synergy to Balance and Climb
Profession (Scribe) synergy to Decipher Script
Profession (Stable hand) synergy to Handle Animal
Profession (Tanner) synergy to Craft (Leather working)
Profession (Teamster) synergy to Handle Animal, hurry teams of animals hitched to large wagons, coaches or other, only
Profession (Woodcutter) synergy to Craft (Carpentry)
I am sure we can come up more like this.