We celebrate Lammas with a fire in our little patio fire pit and we do the whole "Regrets, Farewells, Harvests, Preserves". The highlight for us is writing down our regrets on a piece of paper, saying farewell to them (and to other things in our life that need to be passed) and tossing the papers into the fire (since it's a fire holiday)
We haven't come up with a good way to do harvests/preserves other than chatting about them!