To encourage the kids to play the rule in our house for the summer is no TV/Wii/Computer time for the kids before 7PM (6 in spring and 5 in Winter). So TV is the treat. They have to play or sit around being bored. My kid's favorite day trips won't help you, Museum trip/beach/Bass Pros Shops (love the activities they offer) as you said you're in a rural area. But things that I do to help keep them busy:
Play dates - you can never go wrong with a play date with a good friend.
If there's money in the budget I'll buy a pass to the local rec dept. summer camp. Our's sells them for periods of 10 or 14 days (for a cost of less than $400

). These are random days that we get to pick and the let the kids share them. The program is called Plaground fun and the kids just play random games together. It's so much fun that when offered the choice between that and cub scout camp youngest DS chose rec camp. I'd Check to see if your rec dept has a similar program.
I'm getting a membership at the pool in the next town over - $200/season. It's a town pool but rarely used. Other people I know get a membership at a local lake - $50/season. If going to the lake we prefer to go after 4 when it's free.
Check with the library see what programs they are offering.
Barnes & Noble is offering a summer reading program.
http://www.barnesandnoble.com/summerreading/index.asp I know you said you live in a rural area, but the journal is downloadable and the trip to get the book could make a fun day trip - it could just cost your gas and maybe to make it really special a cookie and juice at their cafe. There is also a list of parent/child activities.
Boys you said? - Nerf Gun Wars and/or Super Soaker Wars or Water Balloon Wars. Get a couple of friends so they can have teams.
The sprinkler.
Build a fort - inside or out. My kids build them with blankets and pillows in the house and left over wood out in the woods (our old fence) - the ones in the woods have now become permanent installations. Which they they use for the aforementioned Nerf/Super Soaker Wars.
Target Dollar Stop - a couple of days ago they had a make your own day camp theme going - things like sand art, etc.
Find a cheap tent at a yard sale - set it up in the yard. A few blankets and some pillows airmattress and Voila -Back yard camp out. Make hot dogs/burgers then s'mores over the grill.
Flashlight tag
Lego Contests - who can build the wildest wackiest creation - this one will keep mine busy for hours.
Do your kids belong to Cub Scouts? Even if they don't check out the electives that they suggest there are some amazing ideas. Taking just one of the ideas and applying some variations I was able to keep 6 boys entertained for 45 minutes with 6 chocolate eggs and some disposable pie pans
http://meritbadge.org/wiki/index.php/Tiger_Cub_Electives
http://meritbadge.org/wiki/index.php/Wolf_Electives
Have a field day - things like long jump, running in place, crab walk, elephant walk, summersalts, etc.
Make paper cup popsicles
Make slushies/frappes/smoothies
Go for a hike.
Google for rail trails/bike paths in your area and go for a family bike ride or walk.
Lowe's sells some great wooden builder kits designed for young kids (bird houses, banks, etc.)
Go Fishing - though usually this is with DH or Grandpa (cause they fish in Grandpa's backyard).
Well, that's it for now. HTH