We plan to take dd to butterfly creek for her second birthday. We'll let people know we're going and can come along if they wish but if they don't it will still be fun.
Otherwise, pools, movies, mini golf etc
Picnic or plan a fun family treasure hunt around your area
Is there a zoo/aquarium/theme park/aquatic fun centre or other fun place to visit in your area?
Or have a special activity at home, like pizza making or other fun food.
Dress up in costumes and make a silly home video.
I have never failed by hiring jumping castles for my kids. They are completely occupied and have a ball and I find that I don't even have to plan too many activities. I usually hire them from http://www.castlecapers.com.au but there are companies everywhere now.
Slumber parties are always cool and usually a little easier to plan as the numbers are smaller. There are people out there who set them all up for you too like https://www.myslumberparty.com.au. They are local to me but there are similar businesses all around the country.