Enjoy Quora as a general distraction to real life, and follow a wide range of topics, particularly about languages and language learning, the culture of European Countries, Politics, Economics, Social Studies, Science & Technology and Sport - and then whatever takes my fancy.
35 year old dude with dual Irish / British nationality, based in the North East of England these days; lived in North Wales, Ireland, London, Galicia (Spain), and Thüringen (Germany) in the past.
The five other extant and officially accepted West Germanic languages[nb1] would be a good start.
Yiddish - I can understand mostly a conversation in Yiddish, especially if they speak with me slowly or give an explanation on any particular words that are cultural. The few grammatical differences like the double negatives also easy to grasp. A German who has never learned any other language would understand Yiddish more than anything else, including Dutch or English.
Of course that's not including written intelligibility, it being the only Germanic language not to use Latin Script.
Low German - the traditional language of what is nowadays Northern Germany, still spoken in rural pockets to certain degrees. Admittedly never something I've studied, but to most Germans wherever they're from, I'd imagine it still to be more intelligible than Dutch is, though for most probably slightly less than Yiddish is.
Frisian - both West Frisian and North Frisian dialects. At least as intelligible as Dutch is for German speakers.
Lëtzebuergisch / Luxembourgish - Effectively is the same dialect as that spoken over the border in the Trier area, only more formalised due to it having become an official language of a country (Luxembourg is sort considered a special case as to whether it's a German-speaking country or not. It is in practice, but the dialect takes precedence over standard German, though standard German is still strong in education and print media, plus television).
Afrikaans - I'd guess intelligibility to be only slightly less than Dutch, and still a bit more than English.
Others - To my own perception as well, although not coming from the pov of an L1 German speaker, and maybe they'd disagree, some of the North Germanic languages even though they are in a separate branch, seem to whilst having less intelligibility to German than Dutch has, have at least as much intelligibility to German than English does, especially Danish and Swedish, and perhaps Norwegian only very marginally less compared to English.
[1] Dialects which are considered ‘German’ but their intelligibility isn't always fully mutual with Hochdeutsch I'm not including, but if someone else said that a strong enough dialect within the DACH countries also counts here, I wouldn't disagree with them. Indeed Lëtzebuergisch especially and Low German is very close to being just that anyhow, only more official than say how Sächsisch, Schwäbisch, Bairisch or many of the Schweizerdeutsch dialects are categorised.
