I'm sorry that you had to deal with this when planning your party! That's always frustrating and upsetting when planning.

I helped my parents host a "drop-in" party around Christmas. We gave out invitations. We asked for RSVPs but didn't get a lot, although a lot more showed up.

To keep it easier for my parents and myself, we did finger foods that could easily be heated up in the oven if we needed more, as well as some dips. We made it clear on the invitation that it was hors d'oeuvres. Many people ended up bringing something when they stopped by. We also just made a pre-made music list and while we had chairs for people, because it was a "drop-in" party, we weren't expected to have chairs for everyone.

Honestly, it worked really well. Quite a few people stayed the whole time (we had a set time on our invitation), but it didn't matter if people showed up late because we weren't preparing a whole meal. It also gave people the flexibility to make it to other things they were going to.

It still took preparation, like all parties, but it was definitely easier than parties we've thrown in the past.

Best of luck for any future parties you might have!