Over the course of the festival, I've saved up 100 tokens. How are they best spent?

It depends on what you're wanting the most, there are a few avatar items in fest shop if you like those (or you can try your hand at the retired summer fest item. It's a bit of a gamble though, because sometimes you get really common items)

You can buy a couple shovels/fishing poles for sylesti seeking if you have some left over items, you want to use, but that's even more of a gamble since you might get nothing.

If you haven't already used them and you're just looking to make some quick money, you can sell them on the trade broker for around 1000 gold each. 100k is a pretty good amount of money to make.

What are the items from the previous summer festivals? I typed "summer" into the trade broker, and got some results, but are there any that go by a different name? I saw some "tropical" and "beach attire" in there. Are those summer festival items, too?
Also, I already added the sun tokens to the shop, so I guess I can't sell them. I read they turn into money eventually, though, so would that be a good option, too?

Yeah, eventually any tokens left in the shop will be given back to you that can no long be used, but are worth 500 gold each, so if nothing else there's that!

And the there are quite a few items that are from previous summer festivals, you can find them all in the wardrobe but I know it's a little hard to navigate right now so here are a few examples:

Umm okay so I was breeding two tyrielle's minion lunamara's (sorry if I spelled that wrong) and none of the offspring are themed.. Why not?

They are purebred themed! Offspring of themed don't retain their name: but if their lineage can be traced back to the themed, then they are counted as 'themed' by the community.

Okay thanks :D