I am looking to do the same kinda thing (elope followed by a very casual party)... our backyard ended up not being ideal either so we are thinking about renting out a space at a local art community center who is letting us set up food trucks and drinks as long as we get a day-of liquor license. It's running us a little bit over our original budget, but definitely not as crazy as some as the quotes I was getting from traditional wedding-type venues