Using the soil in last year's grow bags

either tip it onto the soil or add it to your compost bin 

Put where you had Potatoes or Tomatoes last year (if you grew any outside) - then, if you are rotating your crops, they won't see more Spuds or Toms for the next few years until the rotation comes round again. Or put it on the flower beds.

I mix some 50:50 with sharp horticultural sand and grow Carrots in containers, and some goes into the tubs / urns & hanging baskets (with some whole-season fertiliser - the sort that looks like hundreds-and-thousands)

Thanks for the replies, will put it over my flower borders for the year.