Arrive on 29th Aug'26 evening -
Spend overnight in Rimini
30th Aug'26
The old town in Rimini and especially the Roman bridge can be seen easily in the evening, maybe after dinner in one of the restaurants near the canal. You don't need all day to visit it, it can be done in minutes.
My cousin who lives in Rimini, likes to take me to the restaurant Da Lele, close to the Rimini Medical Center where she works. It's a bit south of the city center, but it's great for fish.
During the day you go to San Marino, or hang out at the beach. The beach is nice and sandy with lots of beach clubs (bagni) where you can rent an umbrella, but the water is not the crystal clear water Sardinia or the Caribbean but I don't remember foul odors all the times (many in my lifetime) when I went.
31st Aug'26 -
I wouldn't bother to go there and spend the night. Just go during the day, preferably on the 30th as suggested above, or, if on the 30th you want to spend the day at the beach, then go on the 31st, but return to the same hotel in Rimini. San Marino is a few hours' thing. There is a great view and a fortress but other than that it's just a tourist traps for German and Russian tourists (although not so many Russians now after the Ukraine war).
My personal choice would be to go to San Marino on the 30th from Rimini as a day trip (skip the beach). Then come back to Rimini when done. San Marino can be visited quickly and there are buses from the Rimini station that take you there frequently. If you go early in the morning, you might still have some beach time in the late afternoon. Then have dinner and take a stroll to the historical center (and the famous Roman Bridge) after dinner.
31st Aug or 1st Sep'26 (depending on when you go to San Marino)
Travel to Desanzano Lake Garda or Peschiera (both have stations). Peschiera is more historic and a UNESCO Heritage site, Desenzano has more nightlife and restaurant scene, so it depends on what you want. Between the two I'd stay in Peschiera.
Actually I prefer the north side of the lake (Malcesine, Riva, Limone, etc.), basically the portion of the lake from Garda town and north. However the train doesn't go there and you'd need a bus from Peschiera up (or also from Verona). There are also passenger ferries that you can use to go up and down the lake from Peschiera, from Desenzano, and practically every town on the lake. The lake is long, and the ferries make stops in many towns, so from the south to the north, it takes some time. In the south lake Sirmione is the most popular destination (crowded when you go), accessible via ferry or bus from both Desenzano or Peschiera. It's pretty and when I went there last time (in July) I had trouble finding parking, but that won't be an issue since you don't drive.
4th Sep'26 -
Depart from Lake Garda and stay overnight in Bergamo
5th Sep'26 -
Spend the day in Bergamo and evening travel for flight back to Manchester..
Or would you suggest spending a night in Bologna or Verona or any
other suggestions and reducing the day in Rimini?
Verona would be a great place to visit on a day trip from Peschiera, if you decide to stay there. No need to change hotels, just stay in Peschiera. If you stay in other towns, like Malcesine or Garda, there are buses directly to Verona. If you stay on the lake, use that base for visiting Verona also, don't bother to change hotels to go to Verona. Changing hotels wastes precious time and Peschiera is just minutes from Verona via train (or also buses)