They may have already bought it. Take them to the garden section and watch what they head for. No need to buy the most expensive items, but cheap tools are often a headache. Ask friends or educated garden center employees for advice. Look for one with a good warranty and keep the receipts.
Gardeners have strange tastes.
A bag of compost may be just what they want. 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.