Formal:
* Scheme
* Strategy
* Design
* Blueprint
* Outline
* Program
* Agenda
* Policy
* Proposal
* Project
* Roadmap
Informal:
* Idea
* Thought
* Intention
* Goal
* Aim
* Objective
* Course of action
* Arrangement
Other:
* Itinerary (for travel plans)
* Syllabus (for educational plans)
* Recipe (for cooking plans)
* Script (for performance plans)
* Map (for directions/routes)
The best word to use will depend on the specific situation and the level of formality you want to convey.