Intermixing crops, the spacing can depend on how fast things mature. For instance, you can plant out cauliflowers, and then plant lettuce seedling in between them, because the cauliflowers mature so slowly that the lettuces will be gone by the time space becomes an issue. Tomatoes and peppers, though, are both summer crops, and if you planted them in each others' gaps, they'd probably compete.

Companion planting: I got a tip that planting carrots in between rows of onions would help discourage carrot rootfly. It seems to have worked -- I spaced the onions a little more widely than I would have normally, I think? I can't think of any other experiments I've done in that line, but I'll chime back in if I remember something.

I think you're probably low on options for things to sow now. Brassicas can be planted out, but if you're starting from seed/set, salads and the alliums are about it. Hmm. A spring variety of carrot (like Nantes - the key is fast maturation) might about do, depending on when frost sets in where you are and if you mind picking a little early.

Good luck!