"In summer we walk more often than in winter."
Translation:Летом мы ходим пешком чаще, чем зимой.
I'm not sure you can. Up to this point in the course, I've only seen comparisons without чем when it's nouns being compared, in which case the 2nd noun is put into genitive case. But in this exercise it's adverbs being compared (летом and зимой), so I suspect that чем is mandatory.