Tomar is small enough to cover entirely on foot. The town centre is compact, and the walk from the train station to the main square takes about five minutes; the walk up to the convent gate from the square is another fifteen minutes, uphill on a cobbled road. Wear shoes with grip — the cobbles are smooth and can be slippery in wet weather.
The aqueduct is about 3 kilometres from the town centre; a bicycle is useful if you want to explore it at leisure. There are informal bike rental options in town, or you can walk along the road to the near end of the aqueduct and back in about an hour.
There is no meaningful public transport within the town. Taxis are available at the station and from the main square.