Don’t worry, you have a couple of options. Rincon de la Vieja National Park is about 1.5 hours from the Riu. It is closer than Miravalles Volcano so a better option for you guys. The trails at Rincon aren’t super easy but not too difficult either. The main trail at Las Pailas ranger station leads to bubbling mud pots and volcanic steam vents and passes through some nice forest where you might see wildlife. The trail is a 2 mile loop but you could always go part of the way and then just turn around. There are also some spas near the park where you could go do a mud bath- the kids would probably like it. We have a post on Rincon with more info, directions, etc.: https://www.twoweeksincostarica.com/rincon-de-la-vieja-national-park/
Your best option for a waterfall is Llanos de Cortes, which is right off the main highway near Bagaces. It’s also about an hour and a half from the Riu and you could combine it with Rincon but it would make for a long day. Probably better to split it up. Llanos de Cortes is worth a visit though. It’s a somewhat steep but short hike down to the waterfall and is a really nice place to spend a couple of hours. This post has directions, etc. to the waterfall: https://www.twoweeksincostarica.com/llanos-de-cortez-waterfall/
Something cool you could combine with the waterfall is Las Pumas Rescue Center, a wildlife rehabilitation center right nearby. Here’s a link with more info: https://www.twoweeksincostarica.com/las-pumas-rescue-center/
Hope that helps. Have a wonderful trip!