Repaint, realign and clean out the gutters. Check the roof, repairing or replacing shingles and flashing where needed. Fix broken windows or screens. Make sure the entry light and doorbell work. Clean out the garage.
Clean houses sell more easily than dirty ones.
Brighten things inexpensively with a fresh coat of paint in neutral, buyer-friendly shades. Consider repainting woodwork instead of hanging new wallpaper. Move out all your excess furniture to make rooms seem as spacious as possible. Clean your closets to make them look bigger. Store off-season clothes, and get rid of the excess.