There is almost nothing to see at the Egelsee, and that is part of its charm. It is a small, reed-fringed bog lake in the rolling Thurgau countryside between Gachnang and Frauenfeld, and under its peat and mud lie the remains of a Neolithic lakeside settlement, timber piles and floor layers preserved for thousands of years because air never reached them. It is one of the many sites that make up the Prehistoric Pile Dwellings around the Alps on the UNESCO list, and unlike the reconstructed villages you find elsewhere, this one has been left in the ground on purpose. Archaeologists consider the soggy earth the best museum there is.
So you come here less for a monument than for the setting. Thurgau is the gentle, agricultural corner of Switzerland that the rest of the country teases as Mostindien, cider India, for its endless orchards and its pear and apple juice. In spring the trees between the farms are white with blossom; in autumn the roadside stalls sell freshly pressed sweet must. The lake itself is ringed with reeds and willows, quiet enough that you will hear frogs and the odd heron lifting off before you see it.
The roads around it suit a relaxed day. Small lanes thread between Niederwil, Gachnang and Hüttwilen, rising and falling over moraine hills left by the Rhine glacier, with views north toward the Seerücken ridge and, on clear days, south to the Alps. Surfaces are good, villages are tidy and speed limits are enforced with Swiss seriousness, so this is not a place for heroics. Tractors and cyclists share the lanes, and the Thur plain to the south can be windy.
Frauenfeld, the cantonal capital, is less than three kilometres away and has everything you need: fuel stations on the edge of town, supermarkets, cafés and a compact old centre around a castle whose stone keep dates back to the Middle Ages. It is a good place for lunch before heading on toward the Untersee or back toward Winterthur. If you want a swim on a hot day, Hüttwilen's little lakes nearby are popular with locals, but the Egelsee itself is a protected wetland, and it is better to leave it to the frogs and the buried village.