Several guidebooks mention Titi Lake as a possible side trip from Larjung or Kalopani on the Annapurna Circuit. But there's more ...
From Kalopani, walk back through Dhampu, but don't drop down onto the river bed. After the buildings there are two small scruffy paths either side of a recently built porter stop. Climb up, go straight through the first set of buildings (where the children will yell "namaste" whilst you're in view).
The track now becomes a wide dirt trail - stay on this, enjoying an almost certainly solitary stroll through peaceful pine forests. Remember to look back occasionally, at magnificent views of the Dhaulagiri icefall. The path then winds through a small village (called Titi, I think) and a few minutes later you reach the lake. In December it should have been renamed Titi Damp Spot. You can drop down to the lake, and the few I met who did this side trip then returned the way they came. However ...
Back on the wide path, follow it round the lake and up to some chortens. There are some really lovely shots of the lake and the icefall from here. At the fork, the left trail leads to Taglung, then to Choyyo, the right trail just to Choyyo. Both villages are hardly ever visited, the locals are very friendly and inquisitive.
To get back to Kalopani, take the Lete path from Choyyo (it heads roughly south past a white building - you may have to ask) - there are splendid views down to the river and across to Nilgiri South. The path descends and skirts round the edge of the ridge. Cross the big metal suspension bridge and turn right - Lete is only a little way away.
The whole trip takes about four hours.