My favorite official campaign setting had a specific focus on these non-delving parts of the game. It was the Birthright campaign setting (link goes to the anniversary page).
The Birthright setting was ripe for regular adventure and provided a continent filled with interesting peoples to meet and places to explore. Where it took off and became different than standard 2E D&D was that it also included rules for "Regency" which was a system that allowed people to rule over nations, temples, merchant houses, or wizards conclaves. The system they used for this became the main focus of what most people saw as the hook for the setting, and it was a fairly complete rules system that laid out the actions a leader might take to conduct business (trades, spying, armies, exchanges, negotiations, building up your own resources, etc.).
It was another layer on top of the game, but it was an effective layer that simulated nation building without making it too hard. There is an official (for 3E) Birthright fansite out there that you can investigate too.
What you really need though are just the rules. I searched them on Ebay and found this copy. Here it is at Amazon too.
The original campaign setting box had all the rules you need for nation building as well as large scale warfare (too simplified for me, but it's there). You wouldn't have to pick up any other product from that campaign setting to get what you need to tweak out yours if you want.
While I never ran Birthright flat out as a "rulership only, no adventure" game, the rules added another layer of meat to the bone for the game and was lots of fun.