Just off the top of my head here are some items you could include:-
a hand fork and trowel
pretty gardening gloves
a kneeler
hand soap
hand cream
packets of seeds
a book about gardening or plants and flowers
plant labellers( not sure if this is what they are really called)
I think I would dress the basket simply with some coloured tissue paper in the bottom, and when choosing the items keep them to a colour theme, arrange the items nicely and tie a pretty ribbon around the rim of the basket.