I'm guessing that since you are looking for things to do onsite, that you are staying onsite. The hotels have activities in the evenings. Movies are shown by the pool on Fridays and sometimes on Saturdays too (at HRH anyway). When we were at HRH last summer, there was a pool party one evening with a DJ and games. It was a hit with the kids, young and old alike. I think RPR also has activities by the pool in the evening. You can go from one hotel to another. On Citywalk, my son likes to spend time in the game area at NBA City. There is also an arcade on the lower level of Motown Cafe and a multiplex cinema on City Walk.
Of course, you should never underestimate the value of a good night's sleep or just relaxing at the hotel while on a theme park vacation. I do not know if you have children or not, but I find that kids do best if they are not overstimulated and have time to just chill, even when you are on the doorstep of all that UO has to offer. For us, problems arise when we try to "do it all" and drag our son (he's 11) from activity to activity to activity without giving him some down time. (not that you asked for parenting advice!)