There's a cute little shag bathroom rug at Ikea... Maybe not.  Seriously though, there's a ton you could do with a hole.   From sandboxes to swimming holes.  But me?  I'd line it with super thick plastic and plant some bamboo in memory of your fallen giant.

Firepits are cool, but make sure the city is ok with them - you probably want to check on ordinances regarding them. From what I see, it looks like you'll have to tell them it's a COOKING firepit.

http://www.ci.stpaul.mn.us/depts/fire/Divisions/FirePrevention/FwksOpenBurngPmtsFireWatchAssmby.htm

Another idea, instead of having an open firepit, is to build a brick stove there. I don't know if you'd want one, but it's an idea.

But if it was me, I would definitely consider building a little patio. You'll want enough room to comfortably seat and grill for your friends, but most of the digging is already done! Just bag up the mulch (whatever you don't need for your other gardens) and leave it on the boulevard with a FREE sign on it. It'll be gone inside a week. Then, dig out the area you want to cover with your patio bricks, offer free beer and pizza for your friends and have yourself a brick-laying party. It should be done within a day or two at most.

You'll need gravel, plastic sheets, and that jack-hammer earth-poundy thing. A good time should be had by all.

you could do a pond......

Ha! that's actually a good idea (although all the wood is gone now - the city won't let you keep diseased wood). I love the thought of sitting around a warm fire on a fall evening.

I might put a fire pit there, and you may have plenty of wood to burn