The majority of what comes home for us are just the cards. A few moms add pencils, erasers or stickers.
We had a really lovely Valentine card come home last year from my son's friend - it was a home made Valentine, a picture of himself made into a card with a cute saying for Valentines. It's still up on our fridge.
Back in the day, I'd hand out little boxes of Valentines Smarties with our older kids and that was always a hit - it's too bad they've curbed that, although I get it.
We've done stickers (they came with the Valentines one year). The tattoo idea is cute (kids love those). We haven't had that before, but I bet those would get used up quickly.
I don't usually do it - I leave it up to the kids to decide (my boys never seemed to care), but this year while standing in line we saw some of those heart shaped red straws and my daughter expressed interest. They were really cheap so I asked her if she'd like to hand them out and she thought that would be fun. I'll tape them to the Valentine cards so they go home with the kids, in case they have drinks at their party. I can just imagine the mess that could make for the teacher (all the kids with funky straws and drinks). She's in grade one.
Another one of my children handed out these neat Lego Valentines - they could be turned into little spinning tops. They were very popular and they made them in the classroom and played with them at lunch. It was just a neat design. I haven't seen them since.
We do have some moms who go to great lengths and that's fine. My kids don't really care - they just like the act of getting cards from their friends and checking them all out.
Oh - as for if it's to girly for your son to give out heart erasers, I don't think so at all. Kids at that age won't care. I should add to, by the time it all make its way home, the erasers have often separated from the cards and my kids can't remember who gave what.