Hello! That sounds like an exciting trip you're planning to the wonderful cities of Stuttgart, Mannheim, and Heilbronn in Germany. Here are some travel tips to help you prepare:

Best Time to Visit:
The best time to visit Stuttgart and the other cities is usually late spring to early autumn, from April to October. During this time, the weather is generally mild and a lot of events and festivals take place.

Route and Accessibility:
All the three cities are well connected via road and rail. From Stuttgart, you can take the InterCity train to Mannheim which is about 38-40 mins journey. From Mannheim, Heilbronn is about an hour away by train.

Accommodation and Booking:
All three cities offer a range of accommodations, from luxury hotels to budget-friendly hostels. It's best to book your accommodation in advance, especially during the high season and during popular events. Booking sites like Booking.com, Expedia, and Airbnb are commonly used.

Clothing and Essentials:
Pack layers to accommodate fluctuating temperatures. Don't forget a rain jacket or umbrella as rain can be unpredictable. Comfortable walking shoes are a must as these cities are best explored on foot. If you're planning to attend any high-end restaurants or events, some smart casual clothing will be essential.

Health and Safety:
Germany is generally safe for tourists, but like anywhere, it's advisable to keep an eye on your belongings, especially in crowded public places. Most residents have a good understanding of English, but a phrasebook or translation app could also prove handy.

Local Etiquette and Respect:
Germans are known for their punctuality and directness, so it's wise to emulate these behaviours during your visit. Always remember to greet people with a friendly ‘Guten Tag’. In restaurants, it's usual to tip between 5-10% if you are pleased with the service.