It really depends on the callout time. Obviously you are pretty limited if you are on airport reserve (I used to sleep, read, watch DVDs (this was before Netflix had much online content), walk around, chat with other  crews, and, if the weather was nice, hit golf balls at the range across the street from the airport.
On regular reserve at the regional level I had 90 minutes, to get there (with a 30 minute drive). That meant I was on a 30 minute leash from my apartment (giving me 30 minutes at home to shower and get dressed and pack last minute stuff). With that in mind I could run errands, go to the gym, meet friends for coffee or meals (if i didn't mind having to leave in the middle) and, if it looked likely I wouldn't get used, go to (cheap... I was a poor starving regional FO) sporting events. I had a roommate at the time and we played lots (and lots) of MarioKart. By the time I sat reserve as a captain, XBox and Call Of Duty were a thing (and I didn't have a roommate anymore) so that replaced Mario.
To try to not feel like I wasted a bunch of time, but not spend money, I used to read free white papers. I started off with aviation related stuff, but moved on to other topics.
On reserve where I'm at now I have a 3 or 4 hour call out, which let's me do pretty much whatever I want during the day as long as I can keep my phone near me while I'm doing it. YouTube has replaced white paper reading, but you've got to fight the rabbit holes. There are also tons of great free online certificate programs if you want to study but not in an official capacity.