Explore downtown! Easy to walk, many cute shops, and grab a brat and beer at one of the many restaurants.
One of the big activities in the summer is floating the icicle river! This company lets you rent a inner tube, drops you off at the entry point, and direct you to pop out of the river at the right spot. The river is quite mellow in the summer and not very deep, but would not recommend this activity for smaller kids.
While there are many wineries downtown, we've been to the tasting room not too far outside of Leavenworth and it was lovely! There are also other wineries nearby, so encourage you to take a tour.
Google: "easy hikes near Leavenworth WA" and you will find a few! Icicle Ridge Trail and Old Pipeline Bend Trail are a couple of easier and accessible hikes. Please be cautious and feel free to let one of us know you'll be hiking in case of an emergency.