Need toy recommendations for smart puppy

Hi all,

I've got a very smart, very food motivated, three-month-old puppy, and I need some ideas for toys to keep him occupied in his pen while I'm working (I work from home). He loves food puzzles -- I give him his meals in a simple rolling one and I've ordered a Buster minicube for when he outgrows that one -- he also has one of those puzzles where he has to move the pieces to find bits of food, which he whips through in no time. He also loves stuffed squeaky toys -- he's a terrier and likes to murder them. He gets plenty of attention and exercise (not to mention puppy socialization class once a week and lots of short training sessions throughout the day), but the only space I have that's big enough to accommodate his pen is next to my work desk, so he really needs to be kept engaged (until he's ready to nap) or he just whines at me. He has a bully stick in there as well -- he goes through about one a week.

I saw plenty of options on the Internet for harder food puzzles and different kinds of pull-apart toys, but I wondered if anyone had any specific recommendations. He does have a Kong, but I keep that for his crate and freeze it with peanut butter when I need to be away for more than an hour or two. He's VERY well behaved in his crate (he can't see my desk from it), and I do put him in there for short periods from time to time when I need to really focus on work, but I don't want to do that often because I don't want him to develop any negative connotations with the crate. Ideally, I'd like to identify three or four good toys I can rotate in when he gets really antsy.

Any recommendations? Any toys I should definitely stay away from?

Eric