It was Traveller and Gamma World around the same time. A DM at my college D&D club was transitioning between AD&D1e, Traveller and Gamma World rules in a campaign, depending upon the current setting. It sounds crazy and at times it definitely was, but the DM was one of the most skilled I ever played at a table with and he managed to pull it off. While a number of the players were experienced war gamers, so learning and adapting to new rules wan't much of a problem. Definitely one of the weirdest and most enjoyable campaigns I've ever played in.
The next non D&D experience was playing Das Schwarze Auge 2e in the early 90s, when a group of German exchange students were being hosted in my neighborhood. It was at a time when I'd left TTRPG play due to raising a young family, so the convenience of having a short campaign hosted next door was a treat and the game play was certainly a new and enjoyable experience. The students only had the rule books in German, so it was a case of learning while the dice were rolling. The Game Master was an excellent teacher though and I was impressed she was running her own adventures, which were quality stuff. I'm still fond of those rules and their Aventuria setting and own many of the latest editions (5e) publications.