DD loves bananas in hers, although they are a *pain* to wash out later. I also tried melon, but that gave her a rash on her face. I've heard apples work well.
My dd loves cantaloupe in the mesh feeder, she freaks when it's finished and I take it from her. I also have put in watermelon but she doesn't seem to like that as much and it's messier than the cantaloupe b/c of all the juice.
We've done peaches and pears; she likes both but they're both rather juicy/messy. I'm glad you asked, because I'm trying to figure out what else to try, too.
Re: What should I put in the mesh feeder?
pears (they're really yummy right now!), ice if she's teething
my friend would put these toddler biscuits in hers when her kids were a little older but still too young to have choking hazard food