First off, last year I did a video for the Gaol Naofa Youtube channel, which takes a look at what Lùnastal's all about and how it can be celebrated:
The first song on the video is a reaping song – that would have been sung as the harvest was being brought in (the rhythm helping people to get into a groove while they cut the crops, as much helping to pass the time) – that's sung by the Scottish folklorist Margaret Bennett, and the music was composed by her son Martyn Bennett. It's a very modern take on an old traditional song, and a sample of a 1920s threshing machine has been used to give the beat that complements the lyrics. The second song is the same again, this time sung with a more traditional arrangement.
Following on from that, since a major theme of Lùnastal is peace, Kathryn's video on the Prophecy of the Morrígan (or Badb's Prayer for Peace) is also worth a watch:
Which could be incorporated into celebrations if you so wish...
For a more in-depth background on Lùnastal/Lúnasa, and some practical ideas, there are some articles I've done over on Tairis that might be worth a read:
If you'd prefer something a little (a lot) shorter, the Festivals page on the Gaol Naofa site might be more to your taste. One of the things on our to do list is to expand the music section, as can be seen by the number of songs that don't yet have a link; quite a few of these are apt for the harvest, though, so they're worth hunting up (you can find them all on Youtube, I think). The craft section also has some useful stuff if you're looking for things to do, and the links include a video showing you how to make a harvest knot (I've yet to try one myself...).
Games and competitive sports are a big part of the celebrations, and there's a big crossover with the kind of games and amusements that were played during wakes – very apt given the roots of Lùnastal as a funeral games, perhaps – so the Death and Burial article back over at Tairis might be useful too.
If you're looking for something nommy then try some cranachan or fraughan cakes... Going fruit picking up on the hills (or shore) is traditional, especially for bilberries (aka fraughans), but you can just use blueberries if you can't find bilberries in the wild... Gooseberries are another fruit that might be collected for the festival, and they can be used to make a delicious gooseberry fool, which is a kind of syllabub dessert (as is cranachan). For those in warmer climes, brambles (or blackberries) might be available by the beginning of August, which can make a good substitute in the absence of bilberries or gooseberries; they're only very rarely ripe so early over here (in my neck of the woods, anyway) in the wild, but supermarkets often start selling them by this point.
Going by the archaeological evidence, it appears that pork is especially appropriate for feasting on, so a good bit of roast pork, or a stew, with some seasonal vegetables – fresh from the garden, if you grow your own, or else locally sourced if you can afford it – could form the main part of your celebratory feast. For savoury treats, cheese-making is traditional, especially a simple soft cheese like crowdie, which is easy to make (in theory! I've yet to master the art myself because I keep managing to overheat the curds). The smell takes a bit of getting used to while you're heating the milk, and that can be a litle off-putting, but the end result really is quite tasty; try rolling the cheese in toasted oatmeal or crushed black pepper to add more flavour. The leftover whey can be used to make some oatmeal bannocks – also traditional to make for the festival (from a Scots persecptive, at least) – to serve with the cheese, or else there are a number of other traditional recipes you might want to hunt up so it doesn't go to waste (F. Marian McNeill's The Scots Kitchen and The Scots Cellar will be useful).
Story-telling is also an appropriate part of the celebrations, along with music and song, which can help set a very convivial atmosphere if you're celebrating as part of a group or with family. You could tell the story of Tailltiu's sacrifice, for example, or take a look at some of the other Dindshenchas tales like the ones for Nás and Carmun. Maire MacNeill records a huge amount of tales in her massive book, The Festival of Lughnasa, and some of them involve Lugh or Crom Cruach, so it's absolutely well-worth getting a hold of if you can. Failing that, why not compose your own stories or poetry?