There are even sources of cheap or free compost or mulch locally. Call for details. Buy what they already use. Liquid fertilizers are nice but may be more appropriate for houseplants. Houseplants are harder to maintain in the winter, but they can brighten the home if the gardener has the time and inclination to care for them.
Spring is a better time to buy houseplants.
It is time now to plant trees and shrubs. Ideally they need to tell you what they want. Many gardeners have a plan as to how they want their garden to look. Buying unwanted plants spoils this plan. Ask them what they want or buy them a gift certificate.