someone would put in complete charge of Nasa and give me a $25 billion/year budget. I'd do the following.

Copy and improve upon the Energia/Vulkan launch system. Make it capable of putting 200 metric tons into Earth orbit in its maximum configuration.

Build a second generation shuttle to launch on Energia

Build a manned capsule/unmanned supply craft to launch on the Energia.

Build a large orbiting space station (dual keel) for a 27 degree 600 kilometer orbit around Earth. It would be home port, shipyard, and testbed for new technologies. This station would serve as the gateway for the exploration and conquest of the solar system.

Build a fully reusable space tug to put people anywhere within one million kilometers of Earth (the moon included) on a short notice. Two of them would be stationed and serviced at the large space station.

Design and build a large deep space exploration vessel (DSEV) to send people anywhere in the solar system. It would be nuclear powered and have a powerful and reliable propulsion system (VASIMR or something similar) that could be easily serviced and maintained from the space station. This craft would be substantial enough to easily protect the crew from space radiation and put them into orbit of any body in the solar system within a maximum travel period of 24 months.

Build landers to be carried by the DSEV. They would be rated for Mars landing. If they can land and take off from Mars, then they could easily do the same for most of the bodies in the solar system. Two of them would be carried by the DSEV.

IMO it's a straightforward step by step plan that could be brought to fruition in a 12-15 year time frame. Each part of the plan would build upon the last, and as a result, basic infrastructure would be put in place that could be used over and over.