We bought a fence --worth every penny. Seriously, if you are planning on having more kids, find the money for a fence. We did ours in parts, so it took two years, but it was well worth it.
I put my kids to work. They have their own rakes, shovels, little buckets to put yard waste or veggies in, etc. They even have garden gloves. And yes, 10 minutes is about their attention span when they are a toddler. My three year old is now willing to work for about 30 minutes. In fact, he and his 5 year old sister cleaned out a flower bed for me this year. It was great!
Nap time is a great time to get serious work done ;-) if you must entertain while working, let the toddler water plants. Toddlers can water for hours without getting bored..... They'll need a bath afterwards, but at least it keeps them occupied! Also, buy a plastic trowel and provide a digging spot in the yard. They can dig for hours too!
And of course, yard toys! Lots of yard toys! Worth every penny....Freecyle or Craigslist are great ways to get free swing sets. I got an old Rainbow set that would cost 15k new for free a few years ago! You have to be patient, be willing to go take it apart yourself, but there are some great finds out there.