Have you been to Como? You could take the 1810 RE to Como S. Giovanni (a Swiss FLIRT train) taking around 40 minutes and come back on a suburbano from S. Giovanni or a regional service from Lago (both run hourly and take about an hour). The Como - Brunate funicular is to be recommended and runs all evening. The town is also worth visiting and there are wooded hills all around. You can also take a boat trip on the lake, though this is expensive.
Alternatively you could continue your journey on the 1810 to Lugano or Bellinzona for more spectacular scenery (the line runs along the Lago di Lugano past mountain scenery from Capolago). There is a funicular from Lugano station to Lugano city centre and there is a very pleasant lakefront. You could also get a bus to Castagnola (in Lugano) and walk to Gandria (a picturesque fishing village) along a boardwalk along cliffs overlooking the lake. There is also a funicular in Lugano (Monte Brè) but it does not run in the evening outside of July/August. You would arrive at Lugano at 1930 and would leave at 2148 (2208 arr. Chiasso, 2216 dep. Chiasso 2321 arr. Porta Garibaldi) or at 2248.
Como would probably be an easier option.
Prices:
4.55€ Milan - Como each way (by S/RE train, EC trains cost 13€)
5.25 € Como - Brunate return on funicular
5.25 € Milano Centrale - Chiasso (4 minutes on from Como, the Swiss border station) each way
Io viaggio ovunque in Lombardia (I travel anywhere in Lombardia) - includes unlimited travel on buses, trains, trams, metros in Milan and Brescia, boats on the Lago d'Iseo and many funiculars/cable cars in Lombardia and train travel only in some neighbouring areas - 15€ one day / 25€ two days / 30 € three days / 40 € one week (month/3 months/year also available)
Chiasso - Lugano: 8.20 CHF / 7.20 € single / 16.40 CHF / 14.40 € day ticket including travel around Lugano (ticket office at Milano Centrale should also be able to sell 5.25 € single to Chiasso and 7.20 € single to Lugano together but you would miss out on the day ticket and there is likely to be a queue, the alternative is to use Chiasso ticket machine during the 8 minute stop there).
Alternatives would be Arona (around an hour from Porta Garibaldi, a pretty town on Lago Maggiore, 5.80 € single, Io viaggio in Lombardia tickets also valid) or Pavia (around half an hour on the S11 from Porta Garibaldi/Repubblica/Porta Venezia or on a regional train to Genova).
Thanks everyone. It was a beautiful day for a trip to Como.