On the front of housing and food supply. Usually after digging a little in the ground, first thing is dedicated farming efforts, meanwhile carving out a living quarters. Usually if I can get that done and start filling it with beds, usually my 3rd migrant wave will be pushing 40 dorfs. I've had my own record of a 63 strong 3rd wave (the first time I did wonder wth do I do now)
Usually if I get hit with huge waves early on, and I've hit a supply of metal. I'll take whoever's got the best weaponsmithing gets me up to 10 axes and 10 picks.
Easy jobs to tide your dorfs over for a bit.
Mass mining (helps strength trains, mining, militia picks use mining as their skill, and picks are dope weapons)
Mass woodcutting (fck elves, trains strength and other good skills for fighting, and I'ma be needing a ton of wood here soon to make weapons and armor for my armies.) - Biome permitting
Mass plant gathering (F00d 4nD B33R?) - Biome permitting
Mass clay gathering (if you have clay, its basically free, and can be gathered quickly by massed dorfs, and easily used to build huge surface structures. Low quality ones, but nonetheless, you can hastily build huge forts in mere weeks)
Large scale farming is often a good way to eat up the time of a lot of dorfs too. Once you have about 4 trained fulltime growers. Start turning out F00d 4nD B33r, and querns (pretty sure this helps strength) and making clothes. Cloth bins and bags are always in great need in your forts (and wooden bins are a waste of firewood for smelters) If really so inclined, you might even step into the dying to make a couple more peoples busywork, and it does help happythoughts.
So many complain about big migrant waves. I personally love them, because it means that my agenda gets pushed up a couple seasons. Yeah you might get an early goblin siege, but I just design fort entrances to be easily defended, or just sealed until I am ready. Or theres always sending a large squad of poorly trained, poorly armored troops out to fight. Always lots of fun.