Single mom, my son celebrated his 3rd and 5th birthdays at WDW and will celebrate his 7th later this year. For his 3rd the grandparents came along and we brought all the presents and did a full party in the room. He was 3, it didn't really matter all that much to him, but it did to my mom. For his 5th it was just the 2 of us. We had already done a full birthday party with friends and family the week prior with all the presents, so I just packed a special outfit as a gift for the actual day and he got Mickey shoes, a Mickey hat, and a shirt that let everyone know it was his birthday.
This year it will be just the 2 of us again. We will do a small party with friends and gifts the week prior again, and he'll just get a couple of small things on the actual day. I'll also let him buy something at the park, and we are doing the After Hours party as well, so it will be a busy birthday. Capping it off, he gets to build a droid the next day and doesn't know about it. So he's more than covered!