If you want a second date
Skip the waiting game. Telling someone you had a good time and suggesting a day is the clearest signal there is, and most people find it attractive rather than too keen.
I had a really good time tonight. I’d like to see you again — are you free Saturday?
Still smiling about the pool game last night. Round two this weekend?
If you’re not sure yet
You don’t have to decide straight away. A warm thank-you keeps the door open without promising anything. If they suggest a second date, it’s fine to say yes to find out.
If you’re not interested
A short, kind, clear message is the respectful option. Thank them, say you didn’t feel a romantic connection, and wish them well. Don’t give reasons that invite a debate, and don’t offer to be friends unless you mean it.
Thanks for tonight — I enjoyed meeting you, but I didn’t feel a romantic spark. I wish you the best.