To seal exterior doors from moisture, while showing off the wood grain, seal them with marine varnish. This oil-based varnish is used by boaters to protect equipment used on the water, so it can certainly hold up to the demands of a door that sees some contact with rain, snow, and humidity.
Painting a room is perhaps the cheapest home improvement product you can carry out. A fresh coat of paint in a different shade or texture can completely transform a room for minimal cost. If your walls are not in the best condition and you don’t have time to re-plaster them before painting, consider hanging wallpaper instead, to avoid drawing attention to the damage.