He doesn't often spend his weekends doing active things

Thank you, Rachel and Happy New Year to you. In Russian we would use the adverb "активно" here which means "in an active way". It also implies different activities, starting from doing sports to participating in political protests.

Great suggestions already, but I'm with Alex in liking your use of 'seldom' ... 'He is seldom very active at the weekend' sounds natural to me. You could also use 'rarely' in place of 'seldom'. However, the word 'active' could apply to any kind of activity - from getting out of bed to running a marathon, even active politically, so it might need some context to make the meaning clearer. There are other options if you want to be more specific: 'He doesn't get out and about much at weekends' or 'He doesn't do much sport on weekends', or even to cover just about everything, 'he doesn't do much at the weekend'. The preposition (or perhaps prep-determiner) usage is also interesting .... I like 'at the weekend' or 'at weekends', but checking in the dictionary I see Americans prefer 'on weekends': http://dictionary.cambridge.org/dictionary/british/weekend

Thank you, Ruthi.

"He is not often very active at the weekends"