What can I put in my fenced yard to entertain these dogs?

I have two beautiful Weimaraners. They are very active and playful. We live in the mountains and lately they have been taking off for hours scaring the crap out of us. One night a few weeks ago they didn’t come back until after dark.
We recently fenced in a little more than half an acre just for them. We built them a really nice doghouse too.
They spend most of their time with me but I know they need to get out and play more than I do so I have been putting them out there for an hour or two in the afternoon. They do pretty good but sometimes they just cry. If they were not in the fenced area they would be on the other side of the mountain.
Can you think of any big sturdy toys I should put out there to keep them happy? This summer I’ll probably put a kiddie swim pool but I need other ideas for now.
Thanks for there input.