Hi:
Kimberly here from lanscape-solutions-for-you.
You can fill your holes with top soil in your yard. There are options of buying bags of top soil, or you can purchase top soil at a nursery or garden center. If you are thinking about filling your holes in the yard, and maybe planting some grass to help with the overall look, I recommend to my clients to get a mixture of black dirt, sand, and saw dust. This is a great combination for a garden or yard. Once you fill in the holes, throw a little grass seed in each area and water. If you are in the northern hemisphere, fall is near and this is a great time to do gardens and lawns. Holes in yards can be very dangerous.
As far as underneath your deck, I would recommend some fill dirt. This doesn't have to be top quality, so you can save a little money. Filling in holes underneath a deck can be tough if the deck is low. One idea if feasible in your budget, is to fill the holes in and put some lattice work around the base of the deck.
Another idea underneath your deck is to fill the holes in with stones. Once again, you don't have to have top quality stones or rocks.
I hope this helps you some, and if you need more suggestions, feel free to contact me here at the website.
If anyone else has others suggestions, go ahead and share them with us.