mil takes the kids one weekend a month and i am running out of believable plans

so this is a good problem to have, i know, i know. mil is amazing, takes the kids the first weekend of every month from friday afternoon to sunday late afternoon, has done since the youngest was like 3. she lives 90 mins away, big house, the kids love it, its incredible. problem is. F39, dh 41, kids 8 and 11. we've been in the LS for about 2 yrs and we use that weekend EVERY MONTH for either a club night, a meet up w another couple, or a little staycation hotel thing. mil obviously thinks were using it to "have a proper weekend off" which is true. she just doesnt know what the weekend off actually looks like. at first it was easy, "oh were just gonna stay in and watch films", "were doing a little hike sunday", whatever. but shes started asking more specific questions?? like "oh which restaurant did you book this time" and "did you see the new exhibition at the [local museum]" and i ran out of plausible-sounding answers about 8 months ago. now im basically inventing a fake hobby for us every month and remembering which lie i told last time. dh thinks im being paranoid and that shes just making polite conversation. but my mil is sharp, like genuinely sharp. and she already once said "you guys seem to have a very busy social life for two ppl who used to never go out" which... fair. how do other ppl handle the routine relative-sitter situation. do you just have ONE big believable thing you do (we go hiking, we go to gigs, we go to a salsa class) and stick to it forever? do you just be vague? do you tell her enough truth that shed stop asking? obvs not telling her the actual truth, shes a lovely woman but no. just need a sustainable cover story system.